Laser apparatus for generating extreme ultraviolet light

ABSTRACT

A system for generating extreme ultraviolet light, in which a target material inside a chamber is irradiated with a laser beam to be turned into plasma, includes a first laser apparatus configured to output a first laser beam, a second laser apparatus configured to output a pedestal and a second laser beam, and a controller connected to the first and second laser apparatuses and configured to cause the first laser beam to be outputted first, the pedestal to be outputted after the first laser beam, and the second laser beam having higher energy than the pedestal to be outputted after the pedestal.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation of U.S. patent applicationSer. No. 15/447,013 filed on Mar. 1, 2017, which is a Divisional of U.S.patent application Ser. No. 14/241,023 filed on Feb. 25, 2014, which isthe U.S. National Phase under 35 U.S.C. § 371 of International PatentApplication No. PCT/IB2012/001717 filed on Sep. 5, 2012, which claimspriority from Japanese Patent Application No. 2011-220911 filed Oct. 5,2011, and Japanese Patent Application No. 2012-135472 filed Jun. 15,2012, the disclosures of which applications are incorporated byreference herein.

BACKGROUND 1. Technical Field

This disclosure relates to a system and a method for generating extremeultraviolet (EUV) light.

2. Related Art

In recent years, semiconductor production processes have become capableof producing semiconductor devices with increasingly fine feature sizes,as photolithography has been making rapid progress toward finerfabrication. In the next generation of semiconductor productionprocesses, microfabrication with feature sizes at 60 nm to 45 nm, andfurther, microfabrication with feature sizes of 32 nm or less will berequired. In order to meet the demand for microfabrication with featuresizes of 32 nm or less, for example, an exposure apparatus is neededwhich combines a system for generating EUV light at a wavelength ofapproximately 13 nm with a reduced projection reflective optical system.

Three kinds of systems for generating EUV light are known in general,including a Laser Produced Plasma (LPP) type system in which plasma isgenerated by irradiating a target material with a laser beam, aDischarge Produced Plasma (DPP) type system in which plasma is generatedby electric discharge, and a Synchrotron Radiation (SR) type system inwhich orbital radiation is used to generate plasma.

SUMMARY

A system for generating extreme ultraviolet light according to oneaspect of this disclosure, in which a target material inside a chamberis irradiated with a laser beam to be turned into plasma, may include afirst laser apparatus configured to output a first laser beam, a secondlaser apparatus configured to output a pedestal and a second laser beam,and a controller connected to the first and second laser apparatuses andconfigured to cause the first laser beam to be outputted first, thepedestal to be outputted after the first laser beam, and the secondlaser beam having higher energy than the pedestal to be outputted afterthe pedestal.

A method for generating extreme ultraviolet light according to anotheraspect of this disclosure, in which a target material inside a chamberis irradiated with a laser beam to be turned into plasma, may includeirradiating a target material with a first laser beam, a second laserbeam, and a third laser beam having energy higher than the second laserbeam.

BRIEF DESCRIPTION OF THE DRAWINGS

Hereinafter, selected embodiments of this disclosure will be describedwith reference to the accompanying drawings. Note that a polarizer inthis specification may be an example of an optical filter.

FIG. 1 schematically illustrates a configuration of an exemplary EUVlight generation system.

FIG. 2 schematically illustrates an exemplary configuration of an EUVlight generation system according to a first embodiment of thisdisclosure.

FIG. 3 shows an example of a waveform of a main pulse laser beam havinga pedestal according to the first embodiment.

FIG. 4 shows an example of a relationship between a pedestal ratio andconversion efficiency according to the first embodiment.

FIG. 5 shows an example of a relationship between pedestal energy andconversion efficiency according to the first embodiment.

FIG. 6 is a flowchart showing an example of an overall operation of apedestal controller according to the first embodiment.

FIG. 7 shows an example of a pedestal control subroutine in Step S103 ofFIG. 6.

FIG. 8 shows an example of a relationship between a pedestal ratio andconversion efficiency used in the description of the pedestal controlsubroutine shown FIG. 7.

FIG. 9 shows a first modification of the pedestal control subroutine inStep S103 of FIG. 6.

FIG. 10 shows an example of a relationship between a pedestal ratio andconversion efficiency used in the description of the pedestal controlsubroutine shown in FIG. 9.

FIG. 11 shows an example of a pedestal ratio calculation subroutine inStep S114 of FIGS. 7 and 9.

FIG. 12 shows an example of a relationship between total energy of amain pulse laser beam and energy of a pedestal of the main pulse laserbeam used in the description of the pedestal ratio calculationsubroutine shown FIG. 11.

FIG. 13 shows an example of a pedestal stabilization subroutine in StepS105 of FIG. 6.

FIG. 14 shows a modification of a pedestal ratio calculation subroutinein Steps S142 and S145 of FIG. 13.

FIG. 15 shows an example of an adjustment necessity determinationsubroutine in Step S106 of FIG. 6.

FIG. 16 shows a first modification of the adjustment necessitydetermination subroutine in Step S106 of FIG. 6.

FIG. 17 shows a second modification of the pedestal control subroutinein Step S103 of FIG. 6.

FIG. 18 shows an example of a relationship between pedestal energy andconversion efficiency used in the description of the pedestal controlsubroutine shown in FIG. 17.

FIG. 19 shows a third modification of the pedestal control subroutine inStep S103 of FIG. 6.

FIG. 20 shows an example of a relationship between pedestal energy andconversion efficiency used in the description of the pedestal controlsubroutine shown in FIG. 19.

FIG. 21 shows an example of a pedestal energy calculation subroutine inStep S314 of FIGS. 17 and 19.

FIG. 22 shows a modification of the pedestal stabilization subroutine inStep S105 of FIG. 6.

FIG. 23 shows a modification of a pedestal energy calculation subroutinein Steps S342 and S345 of FIG. 22.

FIG. 24 shows a second modification of the adjustment necessitydetermination subroutine in Step S106 of FIG. 6.

FIG. 25 shows a third modification of the adjustment necessitydetermination subroutine in Step S106 of FIG. 6.

FIG. 26 schematically illustrates an exemplary configuration of a mainpulse laser apparatus in which an optical shutter is used as a pedestalcontrol device according to the first embodiment.

FIG. 27 shows a waveform of a pulse laser beam at a position (a) of FIG.26.

FIG. 28 shows a waveform of a pulse laser beam at a position (b) of FIG.26.

FIG. 29 shows a waveform of a pulse laser beam at a position (c) of FIG.26.

FIG. 30 schematically illustrates an exemplary configuration of a mainpulse laser apparatus in which an optical shutter and a saturableabsorber device are used collectively as a pedestal control deviceaccording to the first embodiment.

FIG. 31 shows a waveform of a pulse laser beam at a position (a) of FIG.30.

FIG. 32 shows a waveform of a pulse laser beam at a position (b) of FIG.30.

FIG. 33 shows a waveform of a pulse laser beam at a position (c) of FIG.30.

FIG. 34 shows a waveform of a pulse laser beam at a position (d) of FIG.30.

FIG. 35 schematically illustrates an exemplary configuration of a mainpulse laser apparatus in which a Pockels cell in a master oscillator anda saturable absorber device are collectively used as a pedestal controldevice according to the first embodiment.

FIG. 36 shows a waveform of a pulse laser beam at a position (a) of FIG.35.

FIG. 37 shows a waveform of a pulse laser beam at a position (b) of FIG.35.

FIG. 38 shows a waveform of a pulse laser beam at a position (c) of FIG.35.

FIG. 39 schematically illustrates an exemplary configuration of a mainpulse laser apparatus in which a Pockels cell in a master oscillator andan optical shutter are collectively used as a pedestal control deviceaccording to the first embodiment.

FIG. 40 schematically illustrates an exemplary configuration of a mainpulse laser apparatus in which a master oscillator includes at least twosemiconductor lasers according to the first embodiment.

FIG. 41 shows a waveform of a pulse laser beam at a position (a) of FIG.40.

FIG. 42 shows a waveform of a pulse laser beam at a position (b) of FIG.40.

FIG. 43 shows a waveform of a pulse laser beam at a position (c) of FIG.40.

FIG. 44 shows a waveform of a pulse laser beam at a position (d) of FIG.40.

FIG. 45 is a flowchart showing an example of an overall operation of apedestal controller according to a second embodiment of this disclosure.

FIG. 46 shows an example of a pedestal control subroutine in Step S505of FIG. 45.

FIG. 47 shows an example of a relationship between a pedestal ratio andconversion efficiency used in the description of the pedestal controlsubroutine shown in FIG. 46.

FIG. 48 is a flowchart showing an example of an overall operation of apedestal controller according to a modification of the secondembodiment.

FIG. 49 shows an example of a pedestal control subroutine in Step S605of FIG. 48.

FIG. 50 shows an example of a relationship between pedestal energy andconversion efficiency used in the description of the pedestal controlsubroutine shown in FIG. 49.

FIG. 51 illustrates an example of an optical shutter including twopolarizers and a Pockels cell.

FIG. 52 shows an example of a pulse laser beam entering the opticalshutter shown in FIG. 51.

FIG. 53 shows an example of a high-voltage pulse applied to the Pockelscell in the optical shutter shown in FIG. 51.

FIG. 54 shows an example of a pulse laser beam outputted from theoptical shutter when the high-voltage pulse shown in FIG. 53 is appliedto the Pockels cell shown in FIG. 51.

FIG. 55 schematically illustrates an exemplary configuration of a firstmodification of an optical shutter.

FIG. 56 schematically illustrates an exemplary configuration of a secondmodification of an optical shutter.

FIG. 57 schematically illustrates an exemplary configuration of a thirdmodification of an optical shutter.

FIG. 58 schematically illustrates an exemplary configuration of a fourthmodification of an optical shutter.

FIG. 59 schematically illustrates an exemplary configuration of asaturable absorber device in which a concentration of a saturableabsorber gas is adjustable.

FIG. 60 schematically illustrates an exemplary configuration of asaturable absorber device in which an optical path length through asaturable absorber gas is adjustable.

FIG. 61 shows a target irradiated with a pre-pulse laser beam.

FIG. 62 shows a target irradiated with a pre-pulse laser beam, as viewedin a direction perpendicular to the travel direction of the pre-pulselaser beam.

FIG. 63 shows a diffused target, which is generated when a target isirradiated with a pre-pulse laser beam, irradiated with a main pulselaser beam, as viewed in a direction perpendicular to the traveldirection of the main pulse laser beam.

FIG. 64 shows a diffused target irradiated with a main pulse laser beam,as viewed in the travel direction of the main pulse laser beam.

FIG. 65 shows an example of a relationship between conversion efficiencyand a delay time from irradiation of a target with a pre-pulse laserbeam to irradiation with a main pulse laser beam.

FIG. 66 shows plasma of a target material observed 0 μs after a targetis irradiated with a pre-pulse laser beam having a fluence of 480mJ/cm².

FIG. 67 shows a diffused target and plasma of a target material observed0.5 μs after a target is irradiated with a pre-pulse laser beam having afluence of 480 mJ/cm².

FIG. 68 shows a diffused target and plasma of a target material observed1.0 μs after a target is irradiated with a pre-pulse laser beam having afluence of 480 mJ/cm².

FIG. 69 shows a diffused target and plasma of a target material observed1.5 μs after a target is irradiated with a pre-pulse laser beam having afluence of 480 mJ/cm².

FIG. 70 shows plasma of a target material observed 0 μs after a targetis irradiated with a pre-pulse laser beam having a fluence of 96 mJ/cm².

FIG. 71 shows a diffused target and plasma of a target material observed0.5 μs after a target is irradiated with a pre-pulse laser beam having afluence of 96 mJ/cm².

FIG. 72 shows a diffused target and plasma of a target material observed1.0 μs after a target is irradiated with a pre-pulse laser beam having afluence of 96 mJ/cm².

FIG. 73 shows a diffused target and plasma of a target material observed1.5 μs after a target is irradiated with a pre-pulse laser beam having afluence of 96 mJ/cm².

FIG. 74 shows plasma of a target material observed 0 μs after a targetis irradiated with a pre-pulse laser beam having a fluence of 19.5mJ/cm².

FIG. 75 shows a diffused target and plasma of a target material observed0.5 μs after a target is irradiated with a pre-pulse laser beam having afluence of 19.5 mJ/cm².

FIG. 76 shows a diffused target and plasma of a target material observed1.0 μs after a target is irradiated with a pre-pulse laser beam having afluence of 19.5 mJ/cm².

FIG. 77 shows a diffused target and plasma of a target material observed1.5 μs after a target is irradiated with a pre-pulse laser beam having afluence of 19.5 mJ/cm².

FIG. 78 schematically illustrates an exemplary configuration of aregenerative amplifier.

DETAILED DESCRIPTION

Hereinafter, selected embodiments of this disclosure will be describedin detail with reference to the accompanying drawings. The embodimentsto be described below are merely illustrative in nature and do not limitthe scope of this disclosure. Further, configurations and operationsdescribed in each embodiment are not all essential in implementing thisdisclosure. Note that like elements are referenced by like referencenumerals and characters, and duplicate descriptions thereof will beomitted herein. The embodiments of this disclosure will be describedfollowing the table of contents below.

Contents

1. Overview

2. Overview of EUV Light Generation System

2.1 Configuration

2.2 Operation

3. EUV Light Generation System Including Pedestal Control Device: FirstEmbodiment

3.1 Configuration

3.2 Operation

3.3 Relationship between Pedestal of Main Pulse Laser Beam andConversion Efficiency

3.4 Flowcharts

3.4.1 Pedestal Control Flow

3.4.1.1 Control Flow Based on Pedestal Ratio

3.4.1.1.1 Pedestal Control Subroutine

3.4.1.1.2 Pedestal Control Subroutine: First Modification

3.4.1.1.3 Pedestal Ratio Calculation Subroutine

3.4.1.1.4 Pedestal Stabilization Subroutine

3.4.1.1.5 Pedestal Ratio Calculation Subroutine: Modification

3.4.1.1.6 Adjustment Necessity Determination Subroutine

3.4.1.1.7 Adjustment Necessity Determination Subroutine: FirstModification

3.4.1.2 Control Flow Based on Pedestal Energy

3.4.1.2.1 Pedestal Control Subroutine: Second Modification

3.4.1.2.2 Pedestal Control Subroutine: Third Modification

3.4.1.2.3 Pedestal Energy Calculation Subroutine

3.4.1.2.4 Pedestal Stabilization Subroutine: Modification

3.4.1.2.5 Pedestal Energy Calculation Subroutine: Modification

3.4.1.2.6 Adjustment Necessity Determination Subroutine: SecondModification

3.4.1.2.7 Adjustment Necessity Determination Subroutine: ThirdModification

4. Pedestal Control Device

4.1 Optical Shutter

4.2 Optical Shutter and Saturable Absorber Device

4.3 Combination with Pockels Cell in Master Oscillator

4.3.1 Combination with Saturable Absorber Device

4.3.2 Combination with Optical Shutter

4.4 Embodiment Where Master Oscillator Includes At Least TwoSemiconductor Lasers

5. Controlling Energy of EUV Light by Controlling Pedestal of Main PulseLaser Beam:

Second Embodiment

5.1 Configuration

5.2 Operation

5.3 Effect

5.4 Flowcharts

5.4.1 Control Flow Based on Pedestal Ratio

5.4.1.1 Pedestal Control Flow

5.4.1.2 Pedestal Control Subroutine

5.4.2 Control Flow Based on Pedestal Energy

5.4.2.1 Pedestal Control Flow

5.4.2.2 Pedestal Control Subroutine

6 Optical Shutter

6.1 Combination of Pockels Cell and Polarizer

6.2 Variations of Optical Shutter

6.2.1 First Modification

6.2.2 Second Modification

6.2.3 Third Modification

6.2.4 Fourth Modification

7. Saturable Absorber Device

7.1 Adjusting Concentration of Saturable Absorber Gas

7.2 Adjusting Optical Path Length through Saturable Absorber Gas

8. Supplementary Descriptions

8.1 Diffused Target

8.1.1 Generation of Diffused Target

8.2 Relationship Between Delay Time for Main Pulse Laser Beam andConversion Efficiency

8.3 Relationship between Fluence of Pre-pulse Laser beam and Shape ofDiffused Target

8.4 Regenerative Amplifier

1. Overview

In certain embodiments of an EUV light generation system to be describedbelow, a target material may be irradiated with a pre-pulse laser beamto thereby be turned into a diffused target, and the diffused target maybe irradiated with a main pulse laser beam. Such an EUV light generationsystem may include a device configured to control a pedestal of the mainpulse laser beam. By controlling energy of the pedestal of the mainpulse laser beam, energy of EUV light to be generated in theaforementioned EUV light generation system may be controlled.

2. Overview of EUV Light Generation System

2.1 Configuration

FIG. 1 schematically illustrates a configuration of an exemplary LPPtype EUV light generation system. An LPP type EUV light generationapparatus 1 may be used with at least one laser apparatus 3.Hereinafter, a system that includes the EUV light generation apparatus 1and the laser apparatus 3 may be referred to as an EUV light generationsystem 11. As shown in FIG. 1 and described in detail below, the EUVlight generation system 11 may include a chamber 2 and a target supplyunit. The target supply unit may be a target generator 26. The chamber 2may be sealed airtight. The target supply unit may be mounted onto thechamber 2 to, for example, penetrate a wall of the chamber 2. A targetmaterial to be supplied by the target supply unit may include, but isnot limited to, tin, terbium, gadolinium, lithium, xenon, or anycombination thereof.

The chamber 2 may have at least one through-hole or opening formed inits wall, and a pulse laser beam 31 may travel through thethrough-hole/opening into the chamber 2. Alternatively, the chamber 2may have a window 21, through which the pulse laser beam 31 may travelinto the chamber 2. An EUV collector mirror 23 having a spheroidalsurface may, for example, be provided inside the chamber 2. The EUVcollector mirror 23 may have a multi-layered reflective film formed onthe spheroidal surface thereof. The reflective film may include amolybdenum layer and a silicon layer, which are laminated alternately.The EUV collector mirror 23 may have a first focus and a second focus,and may be positioned such that the first focus lies in a plasmageneration region 25 and the second focus lies in an intermediate focus(IF) region 292 defined by the specification of an external apparatus,such as an exposure apparatus 6. The EUV collector mirror 23 may have athrough-hole 24 formed at the center thereof, and the pulse laser beam31 may travel through the through-hole 24 toward the plasma generationregion 25.

The EUV light generation system 11 may further include an EUV lightgeneration controller 5 and a target sensor 4. The target sensor 4 mayhave an imaging function and detect at least one of the presence, thetrajectory, and the position of a target 27.

Further, the EUV light generation system 11 may include a connectionpart 29 for allowing the interior of the chamber 2 to be incommunication with the interior of the exposure apparatus 6. A wall 291having an aperture may be provided inside the connection part 29, andthe wall 291 may be positioned such that the second focus of the EUVcollector mirror 23 lies in the aperture formed in the wall 291.

The EUV light generation system 11 may also include a laser beamdirection control unit 34, a laser beam focusing mirror 22, and a targetcollector 28 for collecting targets 27. The laser beam direction controlunit 34 may include an optical element (not separately shown) fordefining the direction into which the pulse laser beam 31 travels and anactuator (not separately shown) for adjusting the position and theorientation or posture of the optical element.

2.2 Operation

With continued reference to FIG. 1, the pulse laser beam 31 outputtedfrom the laser apparatus 3 may pass through the laser beam directioncontrol unit 34 and be outputted therefrom after having its directionoptionally adjusted. The pulse laser beam 31 may travel through thewindow 21 and enter the chamber 2. The pulse laser beam 31 may travelinside the chamber 2 along at least one beam path from the laserapparatus 3, be reflected by the laser beam focusing mirror 22, andstrike at least one target 27.

The target supply unit may be configured to output the target(s) 27toward the plasma generation region 25 inside the chamber 2. The target27 may be irradiated with at least one pulse of the pulse laser beam 31.Upon being irradiated with the pulse laser beam 31, the target 27 may beturned into plasma, and rays of light 251 including EUV light 252 may beemitted from the plasma. At least the EUV light 252 included in thelight 251 may be reflected selectively by the EUV collector mirror 23.The EUV light 252 reflected by the EUV collector mirror 23 may travelthrough the intermediate focus region 292 and be outputted to theexposure apparatus 6. Here, the target 27 may be irradiated withmultiple pulses included in the pulse laser beam 31.

The EUV light generation controller 5 may be configured to integrallycontrol the EUV light generation system 11. The EUV light generationcontroller 5 may be configured to process image data of the target 27captured by the target sensor 4. Further, the EUV light generationcontroller 5 may be configured to control at least one of the timing atwhich the target 27 is outputted and the direction into which the target27 is outputted. Furthermore, the EUV light generation controller 5 maybe configured to control at least one of the timing at which the laserapparatus 3 oscillates, the direction in which the pulse laser beam 31travels, and the position at which the pulse laser beam 31 is focused.It will be appreciated that the various controls mentioned above aremerely examples, and other controls may be added as necessary.

3. EUV Light Generation System Including Pedestal Control Device: FirstEmbodiment

An EUV light generation system according to a first embodiment of thisdisclosure will now be described in detail with reference to thedrawings. In the description to follow, an EUV light generation system11A in which a target material is irradiated with multiple pulse laserbeams will be illustrated as an example.

3.1 Configuration

FIG. 2 schematically illustrates an exemplary configuration of the EUVlight generation system 11A. As shown in FIG. 2, the EUV lightgeneration system 11A may include a main pulse laser apparatus 3A, ahigh-reflection mirror 341, a dichroic mirror 342, a pre-pulse laserapparatus 40, high-reflection mirrors 401 and 402, a waveform detectionunit 350, a chamber 2A, and an EUV light generation controller 5A.

The main pulse laser apparatus 3A may include a master oscillator 310, apedestal control device 320, amplifiers 331, 332, and 333, and a lasercontroller 301. The laser controller 301 may be configured to controleach of the master oscillator 310, the pedestal control device 320, andthe amplifiers 331 through 333.

The master oscillator 310 may be configured to output a pulse laser beamat a predetermined repetition rate. The pedestal control device 320 maybe configured to transform a waveform of the pulse laser beam from themaster oscillator 310. Each of the amplifiers 331 through 333 maycontain CO₂ gas as a gain medium. An amplified pulse laser beam may beoutputted from the main pulse laser apparatus 3A as a main pulse laserbeam 31. The central wavelength of the main pulse laser beam 31 may beabout 10.6 μm.

The high-reflection mirror 341 and the dichroic mirror 342 mayconstitute a beam delivery unit. The high-reflection mirror 341 may becoated with a film configured to reflect the main pulse laser beam 31with high reflectance. The beam delivery unit may further include anactuator (not separately shown) for adjusting the position and theorientation of the high-reflection mirror 341. The main pulse laser beam31 incident on the high-reflection mirror 341 may be reflected towardthe dichroic mirror 342.

The pre-pulse laser apparatus 40 may be configured to output a pre-pulselaser beam 41 at a central wavelength of about 1.06 μm. The pre-pulselaser apparatus 40 may, for example, be a Yttrium Aluminum Garnet (YAG)laser apparatus. Pulse duration of the pre-pulse laser beam 41 may beabout 5 ns. The pre-pulse laser beam 41 from the pre-pulse laserapparatus 40 may be reflected sequentially by the high-reflectionmirrors 401 and 402 and then be incident on the dichroic mirror 342.Each of the high-reflection mirrors 401 and 402 may be coated with afilm configured to reflect the pre-pulse laser beam 41 with highreflectance. Further, each of the high-reflection mirrors 401 and 402may include an actuator (not separately shown) for adjusting theposition and the orientation of the respective high-reflection mirrors401 and 402.

The dichroic mirror 342 may serve as a beam axis adjuster for adjustingthe beam axes of the main pulse laser beam 31 and the pre-pulse laserbeam 41 entering the chamber 2A. The dichroic mirror 342 may be coatedon a first surface thereof with a film configured to reflect the mainpulse laser beam 31 with high reflectance and transmit the pre-pulselaser beam 41 with high transmittance. The dichroic mirror 342 may becoated on a second surface thereof with a film configured to transmitthe pre-pulse laser beam 41 with high transmittance. The dichroic mirror342 may be positioned such that the main pulse laser beam 31 is incidenton its first surface and the pre-pulse laser beam 41 is incident on itssecond surface. The substrate of the dichroic mirror 342 may, forexample, be formed of diamond.

The chamber 2A may include a window 21, a laser beam focusing opticalsystem 22A, a target generator 26, a target sensor 4, an EUV collectormirror 23, an energy sensor 90, a beam dump 100, and a connection part29.

Each of the main pulse laser beam 31 and the pre-pulse laser beam 41which have entered the chamber 2A through the window 21 may enter thelaser beam focusing optical system 22A. The window 21 may be coated withan anti-reflection film. The laser beam focusing optical system 22A mayinclude a laser beam focusing mirror 71 and a high-reflection mirror 72.The laser beam focusing optical system 22A may further include a movingplate 73, a plate moving device 74, and mirror holders 71 a and 72 a.The mirror holder 72 a may be provided with an automatic tilt mechanism(not separately shown). The laser beam focusing mirror 71 may be anoff-axis paraboloidal mirror. The laser beam focusing mirror 71 may befixed to the moving plate 73 through the mirror holder 71 a. Thehigh-reflection mirror 72 may be fixed to the moving plate 73 throughthe mirror holder 72 a. The plate moving device 74 may be configured tomove the laser beam focusing mirror 71 and the high-reflection mirror 72along with the moving plate 73.

Each of the main pulse laser beam 31 and the pre-pulse laser beam 41which have entered the laser beam focusing optical system 22A may firstbe reflected by the laser beam focusing mirror 71 toward thehigh-reflection mirror 72. The high-reflection mirror 72 may bepositioned to reflect each of the main pulse laser beam 31 and thepre-pulse laser beam 41 toward the plasma generation region 25. Then,each of the main pulse laser beam 31 and the pre-pulse laser beam 41 maybe focused in the plasma generation region 25.

The plate moving device 74 may move the moving plate 73, to therebyadjust the focus position of each of the main pulse laser beam 31 andthe pre-pulse laser beam 41 in the Z-direction. The mirror holder 72 amay adjust a tilt angle of the high-reflection mirror 72, to therebyadjust the focus position of each of the main pulse laser beam 31 andthe pre-pulse laser beam 41 along the XY plane. The above adjustment maybe controlled by the EUV light generation controller 5A, which will bedescribed later.

The target generator 26 may be configured to output targets 27 towardthe plasma generation region 25. The target generator 26 may be providedwith a two-axis moving device (not separately shown). The two-axismoving device may be configured to move the target generator 26, tothereby adjust the position to which the target 27 is supplied.

The target 27 that has reached the plasma generation region 25 may besequentially irradiated with the pre-pulse laser beam 41 and the mainpulse laser beam 31. The pre-pulse laser beam 41 and the main pulselaser beam 31 may strike the target 27 through a through-hole 24 formedin the EUV collector mirror 23. Upon being irradiated with the pre-pulselaser beam 41, the target 27 may be turned into a diffused target. Thisdiffused target may then be irradiated with the main pulse laser beam31, to thereby be turned into plasma. Light 251 including EUV light 252may be emitted from the plasma.

Part of the pre-pulse laser beam 41 and the main pulse laser beam 31that has passed through the plasma generation region 25 may be absorbedby the beam dump 100. The beam dump 100 may be fixed to the chamber 2Athrough a support 101.

The energy sensor 90 may detect energy of the EUV light 252 emitted inthe plasma generation region 25. The detected energy may be inputted tothe EUV light generation controller 5A.

The waveform detection unit 350 may include a beam splitter 351, afocusing lens 352, and a waveform detector 353. The beam splitter 351may reflect a part of the main pulse laser beam 31 and transmit theremaining part. The focusing lens 352 may be positioned to focus themain pulse laser beam 31 reflected by the beam splitter 351 on aphotosensitive surface of the waveform detector 353. The waveformdetector 353 may monitor a waveform of the main pulse laser beam 31imaged on the photosensitive surface. Alternatively, a diffusion platemay be provided in place of the focusing lens 352. The waveform detector353 may then monitor a waveform of the main pulse laser beam 31 diffusedby the diffusion plate. In other embodiments, a plate having athrough-hole may be provided in place of the focusing lens 352. Thewaveform detector 353 may then monitor a waveform of the main pulselaser beam 31 that has passed through the aforementioned through-hole.In yet another embodiment, the main pulse laser beam 31 reflected by thebeam splitter 351 may be directly incident on the photosensitive surfaceof the waveform detector 353. A waveform detected by the waveformdetector 353 may reflect a part of the waveform of the main pulse laserbeam 31. The detected waveform may then be inputted to the EUV lightgeneration controller 5A. Here, the waveform detector 353 may beconfigured to detect a change over time in energy of the main pulselaser beam 31, and may be substituted by any suitable energy sensor aslong as a value reflecting a waveform of the main pulse laser beam 31can be obtained.

The EUV light generation controller 5A may include an EUV lightgeneration position controller 51, a reference clock generator 52, atarget controller 53, a target generation driver 54, a delay circuit 55,and a pedestal controller 56. The EUV light generation positioncontroller 51 may be connected to each of the reference clock generator52, the target controller 53, the pedestal controller 56, and anexposure apparatus controller 61. The EUV light generation positioncontroller 51 may further be connected to each of the main pulse laserapparatus 3A and the pre-pulse laser apparatus 40 through the delaycircuit 55. The target controller 53 may be connected to each of thetarget sensor 4 and the target generation driver 54. The targetgeneration driver 54 may be connected to the target generator 26. Thepedestal controller 56 may be connected to the pedestal control device320 of the main pulse laser apparatus 3A and to the energy sensor 90.

The interior of the chamber 2A may be divided into upstream anddownstream spaces by a partition 80. The plasma generation region 25 maybe set in the downstream space. The partition 80 may serve to reduce theamount of debris of the target material generated in the downstreamspace which enters the upstream space. The partition 80 may have athrough-hole formed therein, through which the main pulse laser beam 31and the pre-pulse laser beam 41 may travel toward the plasma generationregion 25. The partition 80 may be positioned such that the through-holein the partition 80 is aligned with the through-hole 24 in the EUVcollector mirror 23. The EUV collector mirror 23 may be fixed to thepartition 80 through a holder 23 a.

3.2 Operation

An operation of the EUV light generation system 11A shown in FIG. 2 willnow be described. The EUV light generation system 11A may be configuredto operate under the control of the EUV light generation controller 5A.The EUV light generation controller 5A may receive a request from theexposure apparatus controller 61 regarding a position at which the light251 is to be generated or the plasma generation region 25. The EUV lightgeneration controller 5A may then control each component so that thelight 251 is generated in an EUV light generation request positionindicated by the request from the exposure apparatus controller 61.Alternatively, the EUV light generation controller 5A may control eachcomponent so that the EUV light generation request position indicated bythe request from the exposure apparatus controller 61 coincides with theplasma generation region 25.

The EUV light generation position controller 51 may be configured tocontrol the laser beam focusing optical system 22A. The EUV lightgeneration position controller 51 may send driving signals respectivelyto the mirror holder 72 a and the plate moving device 74. The mirrorholder 72 a may control a tilt angle of the high-reflection mirror 72 inθx- and θy-directions in accordance with a driving signal from the EUVlight generation position controller 51. The plate moving device 74 maymove the moving plate 73 in the Z-direction in accordance with a drivingsignal from the EUV light generation position controller 51.

The EUV light generation controller 5A may receive an EUV lightgeneration request signal from the exposure apparatus controller 61requesting generation of the EUV light 252. Upon receiving the EUV lightgeneration request signal, the EUV light generation controller 5A mayinput an EUV light generation request signal to the target controller53. Upon receiving the EUV light generation request signal, the targetcontroller 53 may send an output signal of a target 27 to the targetgenerator 26.

The target sensor 4 may be configured to detect a position and a timingat which the target 27 reaches the plasma generation region 25.Detection results may be inputted to the target controller 53. Thetarget controller 53 may control the two-axis moving device (notseparately shown) of the target generator 26 in accordance with theinputted detection results. Further, the target controller 53 may beconfigured to adjust a delay time in the delay circuit 55 in accordancewith the inputted detection results. The main pulse laser apparatus 3Aand the pre-pulse laser apparatus 40 may be configured to respectivelyoutput the main pulse laser beam 31 and the pre-pulse laser beam 41 attimings defined by the delay time set in the delay circuit 55.

A waveform of the main pulse laser beam 31 may be detected by thewaveform detection unit 350. The waveform detection unit 350 may send adetected waveform to the pedestal controller 56. The pedestal controller56 may carry out a feedback-control on the pedestal control device 320of the main pulse laser apparatus 3A in accordance with the inputtedwaveform of the main pulse laser beam 31 under the control of the EUVlight generation position controller 51.

Energy of the EUV light 252 detected by the energy sensor 90 may beinputted to the pedestal controller 56. The pedestal controller 56 maycarry out a feedback-control on the pedestal control device 320 of themain pulse laser apparatus 3A in accordance with the inputted energy ofthe EUV light 252.

By controlling energy of a pedestal of the main pulse laser beam 31,energy conversion efficiency from the main pulse laser beam 31 to theEUV light 252 may be improved.

3.3 Relationship Between Pedestal of Main Pulse Laser Beam andConversion Efficiency

Here, a relationship between a pedestal of a main pulse laser beam andconversion efficiency will be discussed in detail with reference to thedrawings. Conversion efficiency is a ratio of energy of emitted EUVlight to energy of a pulse laser beam in an LPP type EUV lightgeneration apparatus. FIG. 3 shows an example of a waveform of a mainpulse laser beam having a pedestal. FIG. 4 shows an example of arelationship between a pedestal ratio and conversion efficiency. FIG. 5shows an example of a relationship between pedestal energy andconversion efficiency. Here, a pedestal ratio is a ratio of energy of apedestal to total energy of a main pulse laser beam.

As shown in FIG. 3, a waveform of the main pulse laser beam 31 mayinclude a pedestal 31 p and a peak portion 31 m. The pedestal 31 p may,for example, rise gradually, and the peak portion 31 m may rise inapproximately 100 ns after the rise of the pedestal 31 p. Beam intensityof the pedestal 31 p may be sufficiently small with respect to beamintensity of the peak portion 31 m.

As shown in FIG. 4, where a pedestal ratio is in a range of 1% to 10%,relatively high conversion efficiency of approximately 2.7% to 3.3% isobtained. By varying a pedestal ratio, conversion efficiency may vary.That is, by controlling a pedestal ratio, energy of emitted EUV lightmay be controlled.

As shown in FIG. 5, where energy of a pedestal is in a range of 1 mJ to10 mJ, relatively high conversion efficiency of approximately 2.7% to3.3% is obtained. By varying energy of a pedestal, conversion efficiencymay vary. That is, by controlling energy of a pedestal, energy ofemitted EUV light may be controlled.

3.4 Flowcharts

An operation of the EUV light generation system 11A according to thefirst embodiment will now be described in detail with reference to thedrawings.

3.4.1 Pedestal Control Flow

FIG. 6 is a flowchart showing an example of an overall operation of thepedestal controller according to the first embodiment.

As shown in FIG. 6, the pedestal controller 56 may stand by until itreceives an EUV light generation start signal from the EUV lightgeneration position controller 51 (Step S101; NO). Upon receiving an EUVlight generation start signal (Step S101; YES), the pedestal controller56 may notify the EUV light generation position controller 51 of a startof pedestal control (Step S102). Then, the pedestal controller 56 maycarry out a pedestal control subroutine to control the pedestal controldevice 320 so that a pedestal ratio or energy of a pedestal of the mainpulse laser beam 31 is brought to a desired pedestal ratio or energy(Step S103).

When the control of the pedestal control device 320 is completed, thepedestal controller 56 may notify the EUV light generation positioncontroller 51 of the completion (Step S104). Then, the pedestalcontroller 56 may carry out a pedestal stabilization subroutine tostabilize a pedestal of the main pulse laser beam 31 (Step S105). Here,the main pulse laser beam 31 may be outputted at a predeterminedrepetition rate.

Thereafter, the pedestal controller 56 may carry out an adjustmentnecessity determination subroutine to determine whether or not thepedestal needs to be adjusted (Step S106). Subsequently, the pedestalcontroller 56 may determine whether or not the pedestal needs to beadjusted based on a result of the adjustment necessity determinationsubroutine (Step S107). When the adjustment of the pedestal is needed(Step S107; YES), the pedestal controller 56 may return to Step S102 andrepeat the subsequent steps. On the other hand, when the adjustment ofthe pedestal is not needed (Step S107; NO), the pedestal controller 56may then determine whether or not an EUV light generation pause signalhas been received (Step S108). When an EUV light generation pause signalhas been received (Step S108; YES), the pedestal controller 56 mayterminate the operation. On the other hand, when an EUV light generationpause signal has not been received (Step S108; NO), the pedestalcontroller 56 may return to Step S105 and repeat the subsequent steps.

With the above-described operation, the main pulse laser beam 31 havinga pedestal of a desired pedestal ratio or energy may be outputted stablyfrom the main pulse laser apparatus 3A.

3.4.1.1 Control Flow Based on Pedestal Ratio

Each of the subroutines in the pedestal control flow shown in FIG. 6 maybe carried out using a pedestal ratio (see FIG. 4) as a parameter orusing pedestal energy (see FIG. 5) as a parameter. Subroutines that arecarried out using a pedestal ratio as a parameter will first bediscussed in detail with reference to the drawings.

3.4.1.1.1 Pedestal Control Subroutine

FIG. 7 shows an example of a pedestal control subroutine in Step S103 ofFIG. 6. FIG. 8 shows an example of a relationship between a pedestalratio and conversion efficiency used in the description of the pedestalcontrol subroutine shown in FIG. 7.

With reference to FIG. 7, in the pedestal control subroutine, thepedestal controller 56 may first set “0” in a variable N (Step S111).Then, the pedestal controller 56 may increment the variable N by 1(N=N+1) (Step S112).

Then, the pedestal controller 56 may send a control value P to thepedestal control device 320 (Step S113). As described in further detaillater, the control value P may, for example, include a value of avoltage to be applied to a Pockels cell, a value indicating a timing atwhich the aforementioned voltage is applied. When the pedestal controlsubroutine is carried out for the first time, the smallest or largestcontrol value P=P may be sent as an initial control value to thepedestal control device 320. Thereafter, a control value P=P+(N−1)·ΔPstpmay be sent to the pedestal control device 320 for each preset changeamount ΔPstp. The control value P may continue to be sent to thepedestal control device 320 until the control value P reaches an upperlimit or an lower limit (P=P+(k−1)·ΔPstp) of its measurement range.Here, k may be a natural number and an upper limit of the number ofmeasurement points, and k may be determined in advance through anexperiment.

Subsequently, the pedestal controller 56 may carry out a pedestal ratiocalculation subroutine to calculate a pedestal ratio R (Step S114).Here, a value of the variable N held when the pedestal ratio calculationsubroutine is carried out may be used as a parameter in the pedestalratio calculation subroutine.

Then, the pedestal controller 56 may determine whether or not thevariable N has reached or exceeded the preset upper limit k (Step S115).When the variable N is smaller than the upper limit k (Step S115; NO),the pedestal controller 56 may return to Step S112 and repeat thesubsequent steps. On the other hand, when the variable N has reached orexceeded the upper limit k (Step S115; YES), the pedestal controller 56may obtain a lower limit RL and an upper limit RH of a range withinwhich the pedestal ratio R satisfies required conversion efficiency(Step S116). At this time, a pedestal ratio Rc at which the maximumconversion efficiency CE is obtained may also be determined. Thereafter,the pedestal controller 56 may return to the operation shown in FIG. 6.

As Steps S112 through S115 shown in FIG. 7 are repeated, the k number ofpedestal ratios R and the k number of conversion efficiency CE may beobtained. That is, values R1 through Rk and values CE1 through CEk maybe obtained. Using these values, a relational curve between the pedestalratio R and the conversion efficiency CE as shown in FIG. 8 may beobtained. In FIG. 8, a point (R1, CE1) indicates the lower limit of themeasurement range, and a point (Rk, CEk) indicates the upper limit ofthe measurement range. As shown in FIG. 8, the conversion efficiency CEmay have a peak between the lower limit and the upper limit of themeasurement range of the pedestal ratio R. In that case, a pedestalratio Rc corresponding to the peak in the conversion efficiency CE maybe calculated. Further, when the smallest value CEL of the requiredconversion efficiency CE is set in advance, a range within which a valueof the conversion efficiency CE exceeds the smallest value CEL may beset as a control range of the pedestal ratio R. From this control range,the lower limit RL and the upper limit RH of the control range of thepedestal ratio R may be calculated. The relational curve between thepedestal ratio R and the conversion efficiency CE may, for example, bean approximation curve calculated using the least-square approach.

3.4.1.1.2 Pedestal Control Subroutine: First Modification

The conversion efficiency CE may not have a peak within a measurementrange of the pedestal ratio R. Thus, a pedestal control subroutine in acase where the conversion efficiency CE does not have a peak within themeasurement range of the pedestal ratio R will now be discussed. FIG. 9shows a first modification of the pedestal control subroutine in StepS103 of FIG. 6. FIG. 10 shows an example of a relationship betweenpedestal ratio and conversion efficiency used in the description of thepedestal control subroutine shown in FIG. 9.

As shown in FIG. 9, in the first modification of the pedestal controlsubroutine, Steps S111 through S115, which are similar to Steps S111through S115 shown in FIG. 7, may be carried out. Detailed descriptionthereof will be omitted here. Thereafter, the pedestal controller 56 mayobtain the pedestal ratio Rc corresponding to the maximum value of theconversion efficiency CE within the measurement range. Further, thepedestal ratio R at a point where the conversion efficiency CE is at orabove the minimum value CEL of the required conversion efficiency CE maybe obtained to determine the upper limit RH in the control range (StepS216). Thereafter, the pedestal controller 56 may return to theoperation shown in FIG. 6.

When the conversion efficiency CE does not have a peak in a measurementrange of the pedestal ratio R, Steps S112 through S115 shown in FIG. 9may be repeated, and the k number of pedestal ratios R and the k numberof conversion efficiency CE may be obtained. That is, values R1 throughRk and values CE1 through CEk may be obtained. Using these values, arelational curve between the pedestal ratio R and the conversionefficiency CE as shown in FIG. 10 may be obtained. In FIG. 10, a point(R1, CE1) indicates the lower limit of the measurement range, and apoint (Rk, CEk) indicates the upper limit of the measurement range. Asshown in FIG. 10, the conversion efficiency CE may monotonicallydecrease from the lower limit to the upper limit of the measurementrange of the pedestal ratio R. In that case, the conversion efficiencyCE may be at the highest at the lower limit of the measurement range ofthe pedestal ratio R. Thus, the pedestal ratio R at the lower limit ofthe measurement range may be set as an optimal value Rc. Further, whenthe smallest value CEL of the required conversion efficiency CE is setin advance, a range from the lower limit of the measurement range to apoint where a value of the conversion efficiency CE exceeds the smallestvalue CEL may be set as a control range of the pedestal ratio R. Fromthis control range, the upper limit RH of the control range of thepedestal ratio R may be calculated. The relational curve between thepedestal ratio R and the conversion efficiency CE may, for example, bean approximation curve calculated using the least-square approach.

3.4.1.1.3 Pedestal Ratio Calculation Subroutine

FIG. 11 shows an example of a pedestal ratio calculation subroutine inStep S114 of FIGS. 7 and 9. FIG. 12 shows an example of a relationshipbetween total energy of a main pulse laser beam and energy of a pedestalused in the description of the pedestal ratio calculation subroutineshown in FIG. 11.

As shown in FIG. 11, in the pedestal ratio calculation subroutine, thepedestal controller 56 may first receive a detected waveform of the mainpulse laser beam 31 from the waveform detection unit 350 (Step S121).Then, the pedestal controller 56 may receive detected energy Eeuv of theEUV light 252 from the energy sensor 90 (Step S122).

Subsequently, the pedestal controller 56 may calculate total energy Etof a single pulse from the received waveform of the main pulse laserbeam 31 (Step S123). As shown in FIG. 12, the energy Et may be anintegrated value of energy Ep of a pedestal and energy Em of a peakportion.

Then, the pedestal controller 56 may calculate the energy Ep of thepedestal (Step S124). The energy Ep may be calculated as energy of aportion preceding a rise of the peak portion. Alternatively, the energyEp may be obtained by subtracting the energy Em of the peak portion fromthe total energy Et. The rise of the peak portion may be determinedbased on whether or not the beam intensity has exceeded a predeterminedthreshold value.

Thereafter, the pedestal controller 56 may calculate a pedestal energyratio Rn, where Rn=Ep/Et, with respect to the total energy Et of themain pulse laser beam 31 (Step S125). Here, a value of the variable Nheld when the processing has moved to the pedestal ratio calculationsubroutine may be used as a parameter n. That is, n in the energy ratioRn may be an ordinal number that is the same as the variable N.Subsequently, the pedestal controller 56 may calculate conversionefficiency CEn from the main pulse laser beam 31 to the EUV light 252based on the aforementioned energy Eeuv of the EUV light 252 and thecalculated energy Em of the peak portion (Step S126). Here, n in theconversion efficiency CEn may be an ordinal number that is the same asthe variable N. Thereafter, the pedestal controller 56 may return to thepedestal control subroutine shown in FIG. 7 or 9.

3.4.1.1.4 Pedestal Stabilization Subroutine

In a pedestal stabilization subroutine, the pedestal ratio R may beadjusted accordingly so that the pedestal ratio R approaches thepedestal ratio Rc corresponding to the maximum value of the conversionefficiency CE. FIG. 13 shows an example of a pedestal stabilizationsubroutine in Step S105 of FIG. 6.

With reference to FIG. 13, in the pedestal stabilization subroutine, thepedestal controller 56 may stand by until the waveform of the main pulselaser beam 31 is detected by the waveform detection unit 350 and theenergy of the EUV light 252 is detected by the energy sensor 90 (StepS141; NO). When the waveform of the main pulse laser beam 31 and theenergy of the EUV light 252 are detected (Step S141; YES), the pedestalcontroller 56 may carry out a modification of a pedestal ratiocalculation subroutine (Step S142). The modification of the pedestalratio calculation subroutine may be similar to the pedestal ratiocalculation subroutine described with reference to FIG. 11.

Then, the pedestal controller 56 may calculate a difference ΔR, whereΔR=Rc−R, between the pedestal ratio Rc corresponding to the maximumvalue of the conversion efficiency CE and the pedestal ratio R obtainedin the pedestal ratio calculation subroutine (Step S143). Subsequently,the pedestal controller 56 may send a change amount ΔP of the controlvalue to the pedestal control device 320 so that the difference ΔRdecreases (Step S144). The change amount ΔP may be a preset changeamount ΔPstp or a value calculated in accordance with the difference ΔR.

Then, the pedestal controller 56 may again carry out the modification ofthe pedestal ratio calculation subroutine (Step S145). Subsequently, thepedestal controller 56 may overwrite the current conversion efficiencyCE with the conversion efficiency CE calculated in the modification ofthe pedestal ratio calculation subroutine (CE=CE). Similarly, thecurrent pedestal ratio R may be overwritten with a newly calculatedpedestal ratio R (R=R) (Step S146). The respective values CE and R may,for example, be used in the adjustment necessity determinationsubroutine in Step S106 of FIG. 6. Thereafter, the pedestal controller56 may return to the operation shown in FIG. 6.

3.4.1.1.5 Pedestal Ratio Calculation Subroutine: Modification

FIG. 14 shows the modification of the pedestal ratio calculationsubroutine. The modification of the pedestal ratio calculationsubroutine may be used in the pedestal stabilization subroutinedescribed with reference to FIG. 13.

With reference to FIG. 14, the modification of the pedestal ratiocalculation subroutine may be similar to the pedestal ratio calculationsubroutine shown in FIG. 11. For simplifying the description, only theoperation that differs from that shown in FIG. 11 will be describedbelow.

In the modification of the pedestal ratio calculation subroutine, inSteps S135 and S136, the variable N may not be referenced. That is, theenergy ratio R and the conversion efficiency CE at the time of carryingout the modification of pedestal ratio calculation subroutine may becalculated. Thereafter, the pedestal controller 56 may return to thepedestal control subroutine shown in FIG. 13.

3.4.1.1.6 Adjustment Necessity Determination Subroutine

FIG. 15 shows an example of an adjustment necessity determinationsubroutine in Step S106 of FIG. 6.

With reference to FIG. 15, in the adjustment necessity determinationsubroutine, the pedestal controller 56 may determine whether or not avalue set in the pedestal ratio R falls within a range from the lowerlimit RL inclusive to the upper limit RH inclusive and whether or not avalue set in the conversion efficiency CE is equal to or higher than theminimum value CEL (Step S151). When the pedestal ratio R falls within arange from the lower limit RL inclusive to the upper limit RH inclusiveand the conversion efficiency CE is equal to or higher than the minimumvalue CEL (Step S151; YES), the pedestal controller 56 may determinethat the pedestal does not need adjusting (Step S152). Thereafter, thepedestal controller 56 may return to the operation shown in FIG. 6. Onthe other hand, when the pedestal ratio R does not fall within a rangefrom the lower limit RL inclusive to the upper limit RH inclusive or theconversion efficiency CE is smaller than the minimum value CEL (StepS151; NO), the pedestal controller 56 may determine that the pedestalneed adjusting (Step S153). Thereafter, the pedestal controller 56 mayreturn to the operation shown in FIG. 6.

3.4.1.1.7 Adjustment Necessity Determination Subroutine: FirstModification

When the conversion efficiency CE does not have a peak within ameasurement range of the pedestal ratio R, a modification of theadjustment necessity determination subroutine as described below may becarried out. FIG. 16 shows a first modification of the adjustmentnecessity determination subroutine in Step S106 of FIG. 6.

With reference to FIG. 16, in the first modification of the adjustmentnecessity determination subroutine, the pedestal controller 56 maydetermine whether or not a value set in the pedestal ratio R is equal toor lower than the upper limit RH and whether or not a value set in theconversion efficiency CE is equal to or higher than the minimum valueCEL (Step S251). When the pedestal ratio R is equal to or lower than theupper limit RH and the conversion efficiency CE is equal to or higherthan the minimum value CEL (Step S251; YES), the pedestal controller 56may determine that the pedestal need not adjusting (Step S252).Thereafter, the pedestal controller 56 may return to the operation shownin FIG. 6. On the other hand, when the pedestal ratio R exceeds theupper limit RH or the conversion efficiency falls below the minimumvalue CEL (Step S251; NO), the pedestal controller 56 may determine thatthe pedestal needs adjusting (Step S253). Thereafter, the pedestalcontroller 56 may return to the operation shown in FIG. 6.

3.4.1.2 Control Flow Based on Pedestal Energy

A subroutine where pedestal energy Ep is used as a parameter will now bedescribed in detail with reference to the drawings.

3.4.1.2.1 Pedestal Control Subroutine: Second Modification

FIG. 17 shows a second modification of the pedestal control subroutinein Step S103 of FIG. 6. FIG. 18 shows an example of a relationshipbetween pedestal energy and conversion efficiency used in thedescription of the pedestal control subroutine shown in FIG. 17.

As shown in FIG. 17, in the second modification of the pedestal controlsubroutine, in which the pedestal energy Ep is used as a parameter, anoperation similar to the pedestal control subroutine shown in FIG. 7 maybe carried out. Steps S311 through S315 of FIG. 17 may correspond toSteps S111 through S115 of FIG. 7, and detailed description of StepsS311 through S315 will be omitted here. However, in Step S314, thepedestal controller 56 may carry out a pedestal energy calculationsubroutine, which will be described later, to calculate the pedestalenergy Ep.

As Steps S312 through S315 of FIG. 17 are repeated, a relational curvebetween the pedestal energy Ep and the conversion efficiency CE as shownin FIG. 18 may be obtained. In FIG. 18, a point (Epl, CE1) indicates thelower limit of the measurement range, and a point (Epk, CEk) indicatesthe upper limit of the measurement range. As shown in FIG. 18, theconversion efficiency CE may have a peak between the lower limit and theupper limit of the measurement range of the pedestal energy Ep. In thatcase, pedestal energy Epc corresponding to the peak of the conversionefficiency CE may be calculated. When the smallest value CEL of therequired conversion efficiency CE is set in advance, a range withinwhich a value of the conversion efficiency CE exceeds the smallest valueCEL may be set as a control range of the pedestal energy Ep. From thiscontrol range, the lower limit EpL and the upper limit EpH of thecontrol range of the pedestal energy Ep may be calculated. Therelational curve between the pedestal energy Ep and the conversionefficiency CE may, for example, be an approximation curve calculatedusing the least-square approach.

3.4.1.2.2 Pedestal Control Subroutine: Third Modification

The conversion efficiency CE may not have a peak within a measurementrange of the pedestal energy Ep. Thus, a pedestal control subroutine ina case where the conversion efficiency CE does not have a peak withinthe measurement range of the pedestal energy Ep will be discussed below.FIG. 19 shows a third modification of the pedestal control subroutine inStep S103 of FIG. 6. FIG. 20 shows an example of a relationship betweenpedestal energy and conversion efficiency used in the description of thepedestal control subroutine shown in FIG. 19.

With reference to FIG. 19, in a third modification of the pedestalcontrol subroutine, in which the pedestal energy Ep is used as aparameter, Steps S311 through S315, which are similar to Steps S311through S315 shown in FIG. 17, may be carried out. Then, the pedestalcontroller 56 may obtain the pedestal energy Epc corresponding to themaximum value of the conversion efficiency CE within the measurementrange. Further, the pedestal controller 56 may obtain an upper limit EpHof the pedestal energy Ep at which the required conversion efficiency CEis equal to or higher than the minimum value CEL (Step S416).Thereafter, the pedestal controller 56 may return to the operation shownin FIG. 6.

When the conversion efficiency CE does not have a peak within ameasurement range of the pedestal energy Ep, by repeating Steps S312through S315 of FIG. 19, a relational curve between the pedestal energyEp and the conversion efficiency CE as shown in FIG. 20 may be obtained.In FIG. 20, a point (Epl, CE1) indicates the lower limit of themeasurement range, and a point (Epk, CEk) indicates the upper limit ofthe measurement range. As shown in FIG. 20, the conversion efficiency CEmay monotonically decrease from the lower limit to the upper limit ofthe measurement range of the pedestal energy Ep. In that case, theconversion efficiency CE may be highest at the lower limit of themeasurement range of the pedestal energy Ep. Thus, the pedestal energyEp at the lower limit of the measurement range may be set as an optimalvalue Epc. When the smallest value CEL of the required conversionefficiency CE is set in advance, a range from the lower limit of themeasurement range to a point where the value of the conversionefficiency CE exceeds the smallest value CEL may be set as a controlrange of the pedestal energy Ep. From this control range, the upperlimit EpH of the control range of the pedestal energy Ep may becalculated. The relational curve between the pedestal energy Ep and theconversion efficiency CE may, for example, be an approximation curvecalculated using the least-square approach.

3.4.1.2.3 Pedestal Energy Calculation Subroutine

FIG. 21 shows an example of a pedestal energy calculation subroutine inStep S314 of FIGS. 17 and 19. Here, a relationship between the totalenergy of main pulse laser beam and the energy of the pedestal may, forexample, be the same as that shown in FIG. 12.

With reference to FIG. 21, the pedestal energy calculation subroutine ofthe embodiment shown in FIG. 21 may include steps that are similar tothose in the pedestal ratio calculation subroutine shown in FIG. 11.Thus, only the operations of the pedestal energy calculation subroutineof FIG. 21 that differ from those in the pedestal ratio calculationsubroutine shown in FIG. 11 will be discussed below. Steps S321 throughS323 correspond to Steps S121 through S123 in FIG. 11, and thedescription thereof will be omitted here. In Step S324, the pedestalcontroller 56 may calculate pedestal energy Epn of the main pulse laserbeam 31. Here, a value of the variable N held when the processing hasmoved from the pedestal control subroutine may be used as a parameter n.That is, n in the pedestal energy Epn may be an ordinal number that isthe same as the variable N.

In Step S325, the pedestal controller 56 may calculate energy Em of thepeak portion in the waveform of the main pulse laser beam 31. The energyEm may be energy of a portion of the waveform corresponding to a presetduration after the rise of the peak portion. Alternatively, the energyEm may be obtained by subtracting the pedestal energy Epn from the totalenergy Et of the main pulse laser beam 31. The rise of the peak portionmay be determined based on whether or not the beam intensity hasexceeded a predetermined threshold value.

Step S326 may be similar to Step S126 shown in FIG. 11. Thereafter, thepedestal controller 56 may return to the pedestal control subroutineshown in FIG. 17 or 19.

3.4.1.2.4 Pedestal Stabilization Subroutine: Modification

In a modification of the pedestal stabilization subroutine, the pedestalenergy Ep may be adjusted accordingly so that the pedestal energy Epapproaches the optimal value Epc. FIG. 22 shows the modification of thepedestal stabilization subroutine in Step S105 of FIG. 6.

With reference to FIG. 22, the modification of the pedestalstabilization subroutine of the embodiment shown in FIG. 21, in whichthe pedestal energy Ep is used as a parameter, may include steps thatare similar to those in the pedestal stabilization subroutine shown inFIG. 13. Thus, only the operations of the modification of the pedestalstabilization subroutine of FIG. 21 that differ from those in thepedestal stabilization subroutine shown in FIG. 13 will be discussedbelow. Steps S341 and 342 may be similar to Steps S141 and S142 of FIG.13. However, in Step 342, a modification of the pedestal energycalculation subroutine described with reference to FIG. 23 may becarried out.

In Step S343, the pedestal controller 56 may calculate a difference ΔEp,where ΔEp=Epc−Ep, the difference between the pedestal energy Epccorresponding to the maximum value of the conversion efficiency CE andthe pedestal energy Ep obtained in the modification of the pedestalenergy calculation subroutine. Subsequently, the pedestal controller 56may send a change amount ΔP of the control value P to the pedestalcontrol device 320 so that the difference ΔEp decreases (Step S344). Thechange amount ΔP may be a preset change amount ΔPstp or a valuecalculated in accordance with the difference ΔEp.

Then, the pedestal controller 56 may again carry out the modification ofthe pedestal energy calculation subroutine (Step S345). Thereafter, thepedestal controller 56 may overwrite the current conversion efficiencyCE with the conversion efficiency CE calculated in the modification ofthe pedestal energy calculation subroutine (CE=CE). Similarly, thecurrent energy Ep may be overwritten with newly calculated energy Ep(Ep=Ep) (Step S346). The respective values CE and Ep may, for example,be used in the adjustment necessity determination subroutine in StepS106 of FIG. 6. Thereafter, the pedestal controller 56 may return to theoperation shown in FIG. 6.

3.4.1.2.5 Pedestal Energy Calculation Subroutine: Modification

FIG. 23 shows the modification of the pedestal energy calculationsubroutine. The modification of the pedestal energy calculationsubroutine may be used in the pedestal stabilization subroutinedescribed with reference to FIG. 22.

With reference to FIG. 23, the modification of the pedestal energycalculation subroutine of the embodiment shown in FIG. 23 may includesteps that are similar to those in the pedestal energy calculationsubroutine shown in FIG. 21. As such, only the operations of themodification of the pedestal energy calculation subroutine of FIG. 23that differ from those in the pedestal energy calculation subroutineshown in FIG. 21 will be discussed below.

In the modification of the pedestal energy calculation subroutine, inSteps S334 and S336, the variable N may not be referenced. That is, thepedestal energy Ep and the conversion efficiency CE at the time ofcarrying out the modification of the pedestal energy calculationsubroutine may be calculated. Thereafter, the pedestal controller 56 mayreturn to the pedestal control subroutine shown in FIG. 22.

3.4.1.2.6 Adjustment Necessity Determination Subroutine: SecondModification

FIG. 24 shows a second modification of the adjustment necessitydetermination subroutine in Step S106 of FIG. 6.

With reference to FIG. 24, in the second modification of the adjustmentnecessity determination subroutine, in which the pedestal energy Ep isused as a parameter, the pedestal controller 56 may determine whether ornot a value set in the pedestal energy Ep falls within a range from thelower limit EpL inclusive to the upper limit EpH inclusive and whetheror not a value set in the conversion efficiency CE is equal to or higherthan the minimum value CEL (Step S351). When the pedestal energy Epfalls within a range from the lower limit EpL inclusive to the upperlimit EpH inclusive and the conversion efficiency CE is equal to orhigher than the minimum value CEL (Step S351; YES), the pedestalcontroller 56 may determine that the pedestal need not adjusting (StepS352). Thereafter, the pedestal controller 56 may return to theoperation shown in FIG. 6. On the other hand, when the pedestal energyEp does not fall within a range from the lower limit EpL inclusive tothe upper limit EpH inclusive or the conversion efficiency CE is smallerthan the minimum value CEL (Step S351; NO), the pedestal controller 56may determine that the pedestal need adjusting (Step S353). Thereafter,the pedestal controller 56 may return to the operation shown in FIG. 6.

3.4.1.2.7 Adjustment Necessity Determination Subroutine: ThirdModification

When the conversion efficiency CE does not have a peak within ameasurement range of the pedestal energy Ep, a third modification of theadjustment necessity determination subroutine described below may becarried out. FIG. 25 shows the third modification of the adjustmentnecessity determination subroutine in Step S106 of FIG. 6.

With reference to FIG. 25, in the third modification of the adjustmentnecessity determination subroutine, in which the pedestal energy Ep isused as a parameter, the pedestal controller 56 may determine whether ornot a value set in the pedestal energy Ep is equal to or lower than theupper limit EpH and whether or not a value set in the conversionefficiency CE is equal to or higher than the minimum value CEL (StepS451). When the pedestal energy Ep is equal to or lower than the upperlimit EpH and the conversion efficiency CE is equal to or higher thanthe minimum value CEL (Step S451; YES), the pedestal controller 56 maydetermine that the pedestal does not need adjusting (Step S452).Thereafter, the pedestal controller 56 may return to the operation shownin FIG. 6. On the other hand, when the pedestal energy Ep exceeds theupper limit EpH or the conversion efficiency CE falls below the minimumvalue CEL (Step S451; NO), the pedestal controller 56 may determine thatthe pedestal needs adjusting (Step S453). Thereafter, the pedestalcontroller 56 may return to the operation shown in FIG. 6.

4. Pedestal Control Device

Specific examples of the pedestal control device according to the firstembodiment will now be described in detail with reference to thedrawings.

4.1 Optical Shutter

FIG. 26 schematically illustrates an exemplary configuration of a mainpulse laser apparatus in which an optical shutter is used as a pedestalcontrol device. As shown in FIG. 26, a main pulse laser apparatus 3B mayinclude at least one optical shutter 321 serving as the pedestal controldevice 320. Other configurations may be similar to those of the mainpulse laser apparatus 3A shown in FIG. 2.

The optical shutter 321 may be provided in a beam path of a pulse laserbeam from the master oscillator 310. The optical shutter 321 may beconfigured to vary transmittance therethrough in accordance with thecontrol of the pedestal controller 56. The pedestal controller 56 maycontrol the transmittance of the optical shutter 321 in synchronizationwith a timing at which the pulse laser beam enters the optical shutter321. The timing at which the pulse laser beam enters the optical shutter321 may be detected by an optical sensor (not separately shown). Theoptical sensor may, for example, detect scattered rays of the pulselaser beam outputted from the master oscillator 310.

The optical shutter 321 may be provided in a beam path between themaster oscillator 310 and the amplifier 331. Alternatively, the opticalshutter 321 may be provided downstream from the amplifier 331.

When the pedestal ratio R of the main pulse laser beam 31 is not reducedto a desired value by only a single optical shutter 321, a plurality ofoptical shutters 321 may be used. The plurality of optical shutters 321may be provided in a beam path between the master oscillator 310 and theamplifier 331. However, this disclosure is not limited thereto, and theoptical shutter(s) 321 may be provided in a beam path between theamplifier 331 and the amplifier 332, or in a beam path between theamplifier 332 and the amplifier 333. Alternatively, the opticalshutter(s) 321 may be provided downstream from the amplifier 333.

FIGS. 27 through 29 show waveforms of a pulse laser beam at respectivepositions (a) through (c) in FIG. 26. As shown in FIG. 27, a pulse laserbeam having a waveform Ws which includes a pedestal having a relativelyhigh pedestal ratio R or relatively high pedestal energy Ep may beoutputted from the master oscillator 310. Then, as shown in FIG. 28, thewaveform of the pulse laser beam transmitted through the optical shutter321 may be in a shape where the beam intensity of a front portion of thewaveform Ws is reduced. This waveform may include a pedestal Wp1, wherethe pedestal energy Ep is relatively low, and a peak portion Wm1, whichis a part of the waveform Ws transmitted through the optical shutter 321with high transmittance. Subsequently, as shown in FIG. 29, the waveformof the pulse laser beam amplified in the amplifiers 331 through 333 maybe in a shape where the waveform shown in FIG. 28 is amplified.Similarly to the waveform shown in FIG. 28, the waveform shown in FIG.29 may include a pedestal Wp2, where the pedestal energy Ep isrelatively low, and a peak portion Wm2, which is a part of the waveformWs transmitted through the optical shutter 321 with high transmittance.

4.2 Optical Shutter and Saturable Absorber Device

FIG. 30 schematically illustrates an exemplary configuration of a mainpulse laser apparatus, in which an optical shutter and a saturableabsorber device are collectively used as a pedestal control device. Asshown in FIG. 30, a main pulse laser apparatus 3C may include at leastone optical shutter 321 and at least one saturable absorber devicecollectively serving as the pedestal control device 320. The opticalshutter 321 may be similar to the optical shutter 321 shown in FIG. 26.

The saturable absorber device 322 may be a gas cell containing asaturable absorber gas thereinside. The saturable absorber device 322may be configured such that a concentration of the saturable absorbergas thereinside or an optical path length through the saturable absorbergas is adjustable.

The saturable absorber device 322 may be provided in a beam path of apulse laser beam from the master oscillator 310. The saturable absorberdevice 322 may be provided in a beam path between the master oscillator310 and the amplifier 331. Alternatively, the saturable absorber device322 may be provided downstream from the amplifier 331.

The saturable absorber device 322 may be provided downstream from theoptical shutter 321. With this arrangement, the pedestal energy Ep ofthe pedestal generated by the optical shutter 321 may be adjustedeffectively. Depending on the amplification characteristics of theamplifiers 331 through 333, the gain of the pedestal may be higher thanthe gain of the peak portion. In such a case, a desired pedestal ratiomay not be obtained solely by the optical shutter 321. Accordingly, thepedestal energy Ep may be reduced by the saturable absorber device 322.Then, a desired pedestal ratio R may be achieved. The saturable absorberdevice 322 may, however, be provided upstream from the optical shutter321.

FIGS. 31 through 34 show waveforms of a pulse laser beam at respectivepositions (a) through (d) in FIG. 30. As shown in FIGS. 31 and 32, achange in the waveform before and after the pulse laser beam from themaster oscillator 310 passes through the optical shutter 321 may besimilar to the change in the waveform described with reference to FIGS.27 and 28. Then, as shown in FIG. 33, the waveform of the pulse laserbeam that has passed through the saturable absorber device 322 may be ina shape where the pedestal energy Ep is further reduced or the pedestalratio R is further reduced. At this point, energy of a tail portion ofthe waveform Ws may be reduced by the saturable absorber gas. Thewaveform at this point may include a pedestal Wpl2, where the beamintensity is low, and a peak portion Wml2, where the beam intensity ishigh. Further, as shown in FIG. 34, the waveform of the pulse laser beamamplified in the amplifiers 331 through 333 may be in a shape where thewaveform shown in FIG. 33 is amplified. Similarly to the waveform shownin FIG. 33, this waveform shown in FIG. 34 may include a pedestal Wpl3,where the beam intensity is low, and a peak portion Wml3, where the beamintensity is high.

4.3 Combination with Pockels Cell in Master Oscillator

When a master oscillator includes a Pockels cell, the Pockels cell maybe used as a part of a pedestal control device. Hereinafter, a casewhere the Pockels cell of the master oscillator is used in the pedestalcontrol device will be described with specific examples.

4.3.1 Combination with Saturable Absorber Device

FIG. 35 schematically illustrates an exemplary configuration of a mainpulse laser apparatus, in which a Pockels cell in a master oscillatorand a saturable absorber device are collectively used as a pedestalcontrol device. As shown in FIG. 35, a main pulse laser apparatus 3D mayinclude a master oscillator 311, a high-reflection mirror 317, and thesaturable absorber device 322.

The master oscillator 311 may include a resonator formed byhigh-reflection mirrors 312 and 316, an amplification part 313, apolarization beam splitter 314, and a Pockels cell 315. The Pockels cell315 may change the polarization direction of a passing pulse laser beamin accordance with a voltage applied by the laser controller 301. Thevoltage applied to the Pockels cell 315 by the laser controller 301 maybe controlled by the pedestal controller 56. By adjusting a voltageapplied to the Pockels cell 315 when a pulse laser beam is outputtedfrom the master oscillator 311, a pulse laser beam having a waveform ina shape where the beam intensity of a front portion of the waveform isreduced may be outputted from the master oscillator 311. That is, apulse laser beam of which the pedestal ratio R or the pedestal energy Ephas been adjusted may be outputted from the master oscillator 311.

The pulse laser beam outputted from the master oscillator 311 may bereflected by the high-reflection mirror 317 and enter the saturableabsorber device 322. The saturable absorber device 322 may be similar tothe saturable absorber device 322 shown in FIG. 30. As the pulse laserbeam from the master oscillator 311 passes through the saturableabsorber device 322, the pedestal energy Ep thereof may be adjustedeffectively.

FIGS. 36 through 38 show waveforms of a pulse laser beam at respectivepositions (a) through (c) in FIG. 35. As shown in FIG. 36, a pulse laserbeam having a waveform in which the beam intensity of the front portionis reduced may be outputted from the master oscillator 311. Thiswaveform may include a pedestal Wp21, where the pedestal ratio R or thepedestal energy Ep is relatively low, and a peak portion Wm21, where theenergy Em is relatively high. The pedestal Wp21 may, for example, begenerated by lowering a voltage applied to the Pockels cell 315.Further, the peak portion Wm21 may, for example, be generated by raisinga voltage applied to the Pockels cell 315. When a relatively low voltageis applied to the Pockels cell 315, a change in the polarizationdirection of the pulse laser beam transmitted through the Pockels cell315 may be small. Thus, the beam intensity of the pulse laser beamreflected by the polarization beam splitter 314 may be relatively low.On the other hand, when a relatively high voltage is applied to thePockels cell 315, a change in the polarization direction of the pulselaser beam transmitted through the Pockels cell 315 may be close to 90degrees. Thus, the beam intensity of the pulse laser beam reflected bythe polarization beam splitter 314 may be relatively high. Then, asshown in FIG. 37, the waveform of the pulse laser beam that has passedthrough the saturable absorber device 322 may be in a shape where thepedestal energy Ep is further reduced or the pedestal ratio R is furtherreduced. This pulse waveform may include a pedestal Wp22, where the beamintensity is low, and a peak portion Wm22, where the beam intensity ishigh. At this point, energy of a tail portion of the peak portion Wm22may be reduced by the saturable absorber gas. Further, as shown in FIG.38, the waveform of the pulse laser beam amplified in the amplifiers 331through 333 may be in a shape where the waveform shown in FIG. 37 isamplified. Similarly to the waveform shown in FIG. 37, this waveformshown in FIG. 38 may include a pedestal Wp23, where the beam intensityis low, and a peak portion Wm23, where the beam intensity is high.

4.3.2 Combination with Optical Shutter

FIG. 39 schematically illustrates an exemplary configuration of a mainpulse laser apparatus, in which a Pockels cell in a master oscillatorand an optical shutter are collectively used as a pedestal controldevice. As shown in FIG. 39, a main pulse laser apparatus 3E may besimilar in configuration to the main pulse laser apparatus 3D shown inFIG. 35. However, in the main pulse laser apparatus 3E, the saturableabsorber device 322 may be replaced by the optical shutter 321. Theoptical shutter 321 may be similar to the optical shutter 321 shown inFIG. 26. When a pulse laser beam outputted from the master oscillator311 passes through the optical shutter 321, the pedestal energy Ep ofthe pulse laser beam may be adjusted effectively by adjusting a voltageapplied to the optical shutter 321.

4.4 Embodiment where Master Oscillator Includes at Least TwoSemiconductor Lasers

A master oscillator of a main pulse laser apparatus may include at leasttwo semiconductor lasers. In that case, at least one of thesemiconductor lasers may be used as a pedestal control device.

FIG. 40 schematically illustrates an exemplary configuration of a mainpulse laser apparatus in which a master oscillator includes at least twosemiconductor lasers. A main pulse laser apparatus 3F shown in FIG. 40may be similar in configuration to the main pulse laser apparatus 3Bshown in FIG. 26. However, in the main pulse laser apparatus 3F, themaster oscillator 310 may be replaced by a master oscillator 410.Further, the main pulse laser apparatus 3F may include a regenerativeamplifier 430.

The master oscillator 410 may include semiconductor lasers 411 and 412,and a beam path adjuster 413. Each of the semiconductor lasers 411 and412 may, for example, be a quantum cascade laser. The semiconductorlasers 411 and 412 may be configured to oscillate under the control ofthe laser controller 301. The beam path adjuster 413 may be positionedto adjust the beam paths of the pulse laser beams outputted from therespective semiconductor lasers 411 and 412 to substantially coincidewith each other.

The laser controller 301 may, for example, control the semiconductorlaser 412 to oscillate after the semiconductor laser 411 oscillates. Inthis case, a part of the waveform of the pulse laser beam from thesemiconductor laser 411 may overlap a part of the waveform of the pulselaser beam from the semiconductor laser 412. In other embodiments, thewaveform of the pulse laser beam from the semiconductor laser 411 may betemporally separated from the waveform of the pulse laser beam from thesemiconductor laser 412.

Energy of the pulse laser beam from the semiconductor laser 411 may besubstantially smaller than energy of the pulse laser beam from thesemiconductor laser 412. When these pulse laser beams are combined suchthat the pulse laser beam having lower energy precedes the pulse laserbeam having higher energy, a pulse laser beam that substantiallyincludes a pedestal may be outputted from the master oscillator 410.

The pulse laser beam outputted from the master oscillator 410 may thenbe amplified in the regenerative amplifier 430. The amplified pulselaser beam may enter the optical shutter 321. The optical shutter 321shown in FIG. 40 may be similar to the optical shutter 321 shown in FIG.26. Although in this example, the optical shutter 321 is provideddownstream from the regenerative amplifier 430, the optical shutter 321may be provided in a beam path between the master oscillator 410 and theregenerative amplifier 430. By adjusting a voltage applied to theoptical shutter 321 when the pulse laser beam from the master oscillator410 passes through the optical shutter 321, pedestal energy Ep of thepulse laser beam may be adjusted effectively. Here, the optical shutter321 may be omitted when the pedestal energy Ep obtained by adjusting theenergy of the pulse laser beam from the semiconductor laser 411 isbrought to desired pedestal energy even after the pulse laser beam isamplified in the amplifiers 331 through 333.

FIGS. 41 through 44 show waveforms of a pulse laser beam at respectivepositions (a) through (d) in FIG. 40. As shown in FIG. 41, a waveform ofthe pulse laser beam from the master oscillator 411 may, for example,include a waveform Wp31, where the energy is relatively low, and awaveform Wm31, where the energy is relatively high. The waveform Wp31may, for example, be a waveform of the pulse laser beam from thesemiconductor laser 411. The waveform Wm31 may, for example, be awaveform of the pulse laser beam from the semiconductor laser 412. Then,the pulse laser beam from the master oscillator 410 may be amplified inthe regenerative amplifier 430. In that case, as shown in FIG. 42, awaveform of the pulse laser beam from the regenerative amplifier 430 mayinclude a waveform Wp32, where the energy is relatively low, and awaveform Wm32, where the energy is relatively high. Subsequently, asshown in FIG. 43, a waveform of the pulse laser beam transmitted throughthe optical shutter 321 may include a pedestal Wp33, where the energy ofthe pulse laser beam from the semiconductor 411 is reduced by theoptical shutter 321 and the pedestal ratio R or the pedestal energy Epis relative low, and a peak portion Wm33, where the energy Em isrelatively high. Further, as shown in FIG. 44, a waveform of the pulselaser beam amplified in the amplifiers 331 through 333 may be in a shapewhere the waveform shown in FIG. 43 is amplified. Similarly to thewaveform shown in FIG. 43, this waveform shown in FIG. 44 may include apedestal Wp34, where the pedestal ratio R is relatively low or thepedestal energy Ep is relatively low, and a peak portion Wm34, where theenergy Em is relatively high.

5. Controlling Energy of EUV Light by Controlling Pedestal of Main PulseLaser Beam: Second Embodiment

In the above-described EUV light generation system, the pedestal controldevice is adjusted in order to satisfy the required conversionefficiency. However, this disclosure is not limited thereto. Forexample, energy of the EUV light may be controlled by adjusting thepedestal ratio or the pedestal energy of the main pulse laser beam.Hereinafter, an EUV light generation system configured to control energyof the EUV light by adjusting the pedestal control device will bedescribed in detail as a second embodiment of this disclosure.

5.1 Configuration

The EUV light generation system of the second embodiment may beconfigured similarly to the EUV light generation system 11A of the firstembodiment.

5.2 Operation

The operation of the EUV light generation system of the secondembodiment may be similar to that of the EUV light generation system 11Aof the first embodiment. However, in the second embodiment, targetenergy Eeuvt of the EUV light may be inputted to the EUV lightgeneration controller 5A from an external apparatus, such as theexposure apparatus controller 61. In that case, the EUV light generationcontroller 5A may control the pedestal control device 320 so that theenergy of the emitted EUV light is brought to the target energy Eeuvt.

5.3 Effect

By controlling the pedestal control device 320 to adjust the conversionefficiency CE, the energy of the EUV light 252 may be controlled.Accordingly, the energy of the EUV light 252 may be controlled withoutlargely changing the output power of the main pulse laser apparatus 3A.Thus, variation in a heat load on optical elements provided in a beampath between the main pulse laser apparatus 3A and the plasma generationregion 25 may be reduced. As a result, these optical elements may bethermally stabilized. Accordingly, focusing performance of the mainpulse laser beam 31 may be stabilized, and the output stability of theEUV light 252 may be improved.

5.4 Flowcharts

5.4.1 Control Flow Based on Pedestal Ratio

The operation of the EUV light generation system 11A according to thesecond embodiment may be based on the pedestal ratio R (see FIG. 4) orthe pedestal energy Ep (see FIG. 5). The operation based on the pedestalratio R will first be discussed in detail with reference to thedrawings.

5.4.1.1 Pedestal Control Flow

FIG. 45 is a flowchart showing an example of an overall operation of apedestal controller according to the second embodiment.

With reference to FIG. 45, the pedestal controller 56 may first stand byuntil it receives an target EUV energy Eeuvt of the EUV light 252 froman external apparatus, such as the exposure apparatus controller 61(Step S501; NO). Upon receiving a target EUV energy Eeuvt (Step S501;YES), the pedestal controller 56 may then stand by until it receives atrigger signal from the EUV light generation position controller 51(Step S502; NO). The trigger signal may be a trigger for generating asingle pulse of the EUV light 252. The trigger signals may be inputtedto the pedestal controller 56 at a predetermined repetition rate whilethe EUV light 252 is to be generated.

Upon receiving the trigger signal (Step S502; YES), the pedestalcontroller 56 may receive EUV energy Eeuv of the EUV light 252 detectedby the energy sensor 90 (Step S503). Subsequently, the pedestalcontroller 56 may carry out a pedestal ratio calculation subroutine tocalculate the pedestal ratio R (Step S504). The pedestal ratiocalculation subroutine may be similar to the modification of thepedestal ratio calculation subroutine described with reference to FIG.13.

Then, the pedestal controller 56 may carry out a pedestal controlsubroutine to control the pedestal control device 320 so that thepedestal of the main pulse laser beam 31 achieves a desired pedestalratio R (Step S505).

Thereafter, the pedestal controller 56 may determine whether or notgeneration of the EUV light 252 is to be stopped (Step S506). Thisdetermination may, for example, be made in the pedestal controlsubroutine in Step S505.

When generation of the EUV light 252 is to be continued (Step S506; NO),the pedestal controller 56 may return to Step S502 and repeat thesubsequent steps. On the other hand, when generation of the EUV light252 is to be stopped (Step S506; YES), the pedestal controller 56 mayterminate the operation.

With the above-described operation, the pedestal ratio R of the mainpulse laser beam 31 may be adjusted in accordance with the target EUVenergy Eeuvt. As a result, the target EUV energy Eeuvt may be achievedwithout largely changing the output power of the main pulse laser beam31.

5.4.1.2 Pedestal Control Subroutine

FIG. 46 shows an example of a pedestal control subroutine in Step S505of FIG. 45. FIG. 47 shows an example of a relationship between apedestal ratio and conversion efficiency used in the description of thepedestal control subroutine shown in FIG. 46.

With reference to FIG. 46, in the pedestal control subroutine, thepedestal controller 56 may calculate a difference ΔE, whereΔE=Eeuv-Eeuvt, between the detected EUV energy Eeuv and the target EUVenergy Eeuvt (Step S511). Then, the pedestal controller 56 may calculatea change amount ΔR of the pedestal ratio R corresponding to thecalculated difference ΔE (Step S512). In order to calculate the changeamount ΔR from the difference ΔE, a relationship between a pedestalratio Rn and the total energy Et of the main pulse laser beam to be usedto obtain a relationship between the pedestal ratio R and the conversionefficiency CE shown in FIG. 47 may be used. The relationship between thepedestal ratio Rn and the total energy Et of the main pulse laser beammay be obtained by carrying out the pedestal control subroutinedescribed with reference to FIG. 7 or 9. Subsequently, the pedestalcontroller 56 may calculate a corrected pedestal ratio R from thecurrent pedestal ratio R and the calculated change amount ΔR, (R=R+ΔR)(Step S513).

Then, the pedestal controller 56 may determine whether or not thecalculated pedestal ratio R falls within a monotonic decrease region ofthe conversion efficiency CE in a measurement range of the pedestalratio R (Step S514). The monotonic decrease region of the conversionefficiency CE may be a region in which the conversion efficiency CEdecreases relatively monotonically with respect to the increase in thepedestal ratio R, as shown in FIG. 47. The aforementioned determinationmay be made based on a value of the pedestal ratio R. Here, therelational curve between the pedestal ratio R and the conversionefficiency CE as shown in FIG. 47 may preferably be obtained in advance.This relationship may be obtained by carrying out the pedestal controlsubroutine described with reference to FIG. 7 or 9. In otherembodiments, the relationship between the pedestal ratio R and theconversion efficiency CE obtained in advance through an experiment maybe stored and referenced accordingly.

When the pedestal ratio R falls within the monotonic decrease region ofthe conversion efficiency CE (Step S514; YES), the pedestal controller56 may calculate a control value P of the pedestal control device 320 toachieve the desired pedestal ratio R (Step S515). Thereafter, thepedestal controller 56 may return to the operation shown in FIG. 45. Onthe other hand, when the pedestal ratio R does not fall within themonotonic decrease region of the conversion efficiency CE (Step S514;NO), the pedestal controller 56 may instruct the EUV light generationcontroller 5A to terminate the control of the pedestal to control theenergy of the EUV light 252 (Step S516). Thereafter, the pedestalcontroller 56 may return to the operation shown in FIG. 45.

5.4.2 Control Flow Based on Pedestal Energy

An operation based on pedestal energy will now be described in detailwith reference to the drawings.

5.4.2.1 Pedestal Control Flow

FIG. 48 is a flowchart showing an example of an overall operation of apedestal controller according to a modification of the secondembodiment.

The pedestal control flow as shown in FIG. 48, in which the pedestalenergy Ep is used as a parameter, may include steps that are similar tothose of the pedestal control flow described with reference to FIG. 45.Thus, only the operations of the pedestal control flow of FIG. 48 thatdiffer from those in the pedestal control flow shown in FIG. 45 will bediscussed below. Steps S601 through S603 may be similar to Steps S501through S503 of FIG. 45. In Step S604, the pedestal controller 56 maycarry out a pedestal energy calculation subroutine to calculate thepedestal energy Ep. The pedestal energy calculation subroutine in StepS604 may be similar to the modification of the pedestal energycalculation subroutine described with reference to FIG. 22.

Then, the pedestal controller 56 may carry out a pedestal controlsubroutine to control the pedestal control device 320 so that a pedestalof the main pulse laser beam 31 achieves desired pedestal energy Ep(Step S605).

Step S606 of FIG. 48 may be similar to Step S506 of FIG. 45. However,when the determination is NO in Step S606, the pedestal controller 56may return to Step S602 and repeat the subsequent steps.

With the above-described operation, the pedestal energy Ep of the mainpulse laser beam 31 may be controlled in accordance with the target EUVenergy Eeuvt. As a result, the target EUV energy Eeuvt may be achievedwithout largely changing the output power of the main pulse laser beam31.

5.4.2.2 Pedestal Control Subroutine

FIG. 49 shows an example of a pedestal control subroutine in Step S605of FIG. 48. FIG. 50 shows an example of a relationship between pedestalenergy and conversion efficiency used in the description of the pedestalcontrol subroutine shown in FIG. 49.

With reference to FIG. 49, in the pedestal control subroutine, thepedestal controller 56 may calculate a difference ΔE, whereΔE=Eeuv-Eeuvt, between the detected EUV energy Eeuv and the target EUVenergy Eeuvt (Step S611). Then, the pedestal controller 56 may calculatea change amount ΔEp of the pedestal energy Ep corresponding to thecalculated difference ΔE (Step S612). Subsequently, the pedestalcontroller 56 may calculate corrected pedestal energy Ep, whereEp=Ep+ΔEp, from the current pedestal energy Ep and the calculated changeamount ΔEp (Step S613).

Then, the pedestal controller 56 may determine whether or not thecalculated pedestal energy Ep falls within a monotonic decrease regionof the conversion efficiency CE in a measurement range of the pedestalenergy Ep (Step S614). The monotonic decrease region of the conversionefficiency CE may be a region in which the conversion efficiency CEdecreases relatively monotonically with respect to the increase in thepedestal energy Ep, as shown in FIG. 50. The above determination may bemade based on a value of the pedestal energy Ep. Here, a relationalbetween the pedestal energy Ep and the conversion efficiency CE as shownin FIG. 50 may preferably be obtained in advance. This relationship maybe obtained by carrying out the pedestal control subroutine describedwith reference to FIG. 17 or 19. In other embodiments, a relationshipbetween the pedestal energy Ep and the conversion efficiency CE obtainedin advance through an experiment may be stored and referencedaccordingly.

When the pedestal energy Ep falls within the monotonic decrease regionof the conversion efficiency CE (Step S614; YES), the pedestalcontroller 56 may calculate a control value P of the pedestal controldevice 320 to achieve the desired pedestal energy Ep (Step S615).Thereafter, the pedestal controller 56 may return to the operation shownin FIG. 48. On the other hand, when the pedestal energy Ep does not fallwithin the monotonic decrease region of the conversion efficiency CE(Step S614; NO), the pedestal controller 56 may instruct the EUV lightgeneration controller 5A to terminate the control of the pedestal tocontrol of the energy of the EUV light 252 (Step S616). Thereafter, thepedestal controller 56 may return to the operation shown in FIG. 48.

6. Optical Shutter

The optical shutter of the above-described embodiments will now bedescribed with specific examples.

6.1 Combination of Pockels Cell and Polarizer

FIG. 51 illustrates an exemplary configuration of an optical shutterwhich includes two polarizers and a Pockels cell. A Pockels celltypically has a responsiveness in the order of a few nanoseconds and isconsidered to be suitable as an optical shutter in a laser apparatuswhere high-speed switching is required.

In the configuration shown in FIG. 51, a polarizer 501 may transmit aY-polarization component of a laser beam incident thereon and block anX-polarization component thereof by reflecting or absorbing theX-polarization component. On the other hand, a polarizer 502 maytransmit an X-polarization component of a laser beam incident thereonand block a Y-polarization component thereof by reflecting or absorbingthe Y-polarization component. In this way, the polarizers 501 and 502may be arranged to transmit different polarization components. In thisexample, the polarizers 501 and 502 may be arranged so that thepolarization direction of the laser beam transmitted through thepolarizer 501 differs by 90 degrees from the polarization direction ofthe laser beam transmitted through the polarizer 502.

A high-voltage pulse may be applied to a Pockels cell 503 by ahigh-voltage power source 504 under the control of the pedestalcontroller 56. The Pockels cell 503 may, for example, rotate thepolarization direction of the entering laser beam while the high-voltagepulse is applied thereto. In this example, a high-voltage pulse at avalue for rotating the polarization direction of the entering laser beamincident beam by a predetermined amount may be applied to the Pockelscell 503 by the high-voltage power source 504.

When a pulse laser beam L1 containing largely a Y-polarization componentoutputted from the master oscillator 310 enters the optical shutter 321configured as described above, the pulse laser beam L1 may first beincident on the polarizer 501. The polarizer 501 may transmit aY-polarization component of the pulse laser beam L1. The pulse laserbeam L1 transmitted through the polarizer 501 may then enter the Pockelscell 503.

When a high-voltage pulse is not applied to the Pockels cell 503, thepulse laser beam L1 may be outputted from the Pockels cell 503 withouthaving the polarization direction thereof rotated. The outputted pulselaser beam L1 may then be incident on the polarizer 502, and thepolarizer 502 may block the pulse laser beam L1 in this case. As aresult, the pulse laser beam L1 may be blocked by the optical shutter321.

Meanwhile, when a high-voltage pulse is applied to the Pockels cell 503,the polarization direction of the pulse laser beam L1 entering thePockels cell 503 may be rotated by a predetermined amount. The outputtedpulse laser beam L1 may then be incident on the polarizer 502, and thepolarizer 502 may transmit an X-polarization component of the pulselaser beam L1. As a result, a part of the pulse laser beam L1 may beoutputted from the optical shutter 321.

As shown in FIG. 52, a pulse laser beam L1 having a pulse duration ofabout 20 ns may enter the optical shutter 321. Then, as shown in FIG.53, a high-voltage pulse of a duration in which a temporal jitter of thepulse laser beam L1 is absorbed may be applied to the Pockels cell 503.For example, when the pulse duration of the pulse laser beam L1 is 20ns, and the temporal jitter is 10 ns, the duration of the high-voltagepulse G1 may be approximately 40 ns.

Further, the high-voltage pulse G1 may have a step-like pulse shape inwhich a voltage of a portion where a pedestal Lp is to be generated fromthe pulse laser beam L1 is low and a voltage of the remaining portion ishigh to generate a peak portion Lm. By applying a high-voltage pulsehaving such a pulse shape to the Pockels cell 503, a waveform of thepulse laser beam L1 may be transformed into such a waveform thatcontains the pedestal Lp and the peak portion Lm, as shown in FIG. 54.

In this example, the polarizers 501 and 502 are arranged such that thepolarization direction of the pulse laser beam L1 transmitted throughthe polarizer 501 differs by 90 degrees from that of the pulse laserbeam L1 transmitted through the polarizer 502. However, this disclosureis not limited thereto. For example, the polarizers 501 and 502 may bearranged such that the polarization direction of the pulse laser beam L1transmitted through the polarizer 501 may be the same as that of thepulse laser beam L1 transmitted through the polarizer 502. In that case,while a high-voltage pulse is not applied to the Pockels cell 503, theoptical shutter 321 may transmit the pulse laser beam L1.

6.2 Variations of Optical Shutter

6.2.1 First Modification

FIG. 55 schematically illustrates an exemplary configuration of anoptical shutter of a first modification. In an optical shutter 321-1,reflective polarizers 511 and 512 may, for example, be used in place ofthe transmissive polarizers 501 and 502. A polarizer such as anabsorbing thin-film reflector (ATFR) may be used for each of thepolarizers 511 and 512. Even with such a configuration, a similarfunction to that of the optical shutter 321 shown in FIG. 51 may beachieved. In FIG. 55, the high-voltage power source 504 is omitted.

6.2.2 Second Modification

FIG. 56 schematically illustrates an exemplary configuration of anoptical shutter of a second modification. As shown in FIG. 56, in anoptical shutter 321-2, four reflective polarizers 521 through 524 may beprovided upstream from the Pockels cell 503, and four reflectivepolarizers 525 through 528 may be provided downstream from the Pockelscell 503. An ATFR may be used for each of the polarizers 521 and 528.The polarizers 521 through 524 may, for example, be arranged to reflecta Y-polarization component of the pulse laser beam L1 and absorb theother polarization component thereof. The polarizers 525 through 528may, for example, be arranged to reflect an X-polarization component ofthe pulse laser beam L1 and absorb the other polarization componentthereof. When a plurality of polarizers that reflects the samepolarization component and absorbs the other polarization component isprovided upstream and downstream from the Pockels cell 503,respectively, the total absorptance of a polarization component to beabsorbed may be increased. Thus, the purity of a given polarizationcomponent may be increased.

6.2.3 Third Modification

FIG. 57 schematically illustrates an exemplary configuration of anoptical shutter of a third modification. As shown in FIG. 57, an opticalshutter 321-3 may include two Pockels cells 503 a and 503 b. Each of thePockels cells 503 a and 503 b may be similar to the Pockels cell 503 ofFIG. 55 or 56. The Pockels cells 503 a may be provided upstream from thePockels cell 503 b. Reflective polarizers 531 and 532 provided upstreamfrom the Pockels cell 503 a and reflective polarizers 539 and 540provided downstream from the Pockels cell 503 b may, for example,reflect a Y-polarization component of the pulse laser beam L1 and absorbthe other polarization component. Reflective polarizers 534 through 537may be provided in a beam path between the Pockels cell 503 a and thePockels cell 503 b. The polarizers 534 through 537 may, for example,reflect a Z-polarization component of the pulse laser beam L1 and absorbthe other polarization component. A high-reflection mirror 533 may beprovided downstream from the Pockel cell 503 a and a high-reflectionmirror 538 may be provided upstream from the Pockels cell 503 b. Whenthe plurality of Pockels cells 503 a and 503 b are used, totalabsorptance of a polarization component to be absorbed may be increased.Thus, the purity of a certain polarization component may be increased.

6.2.4 Fourth Modification

FIG. 58 schematically illustrates an exemplary configuration of anoptical shutter of a fourth modification. An optical shutter 321-4 asshown in FIG. 58 may be similar in a configuration to the opticalshutter 321-1 shown in FIG. 55, but may differ in that the polarizers511 and 512 may respectively be provided with cooling devices 551. Acooling medium supplied from the cooling device 551 may flow through aflow channel 552 and into an internal flow channel of each of thepolarizers 511 and 512. Each of the polarizers 511 and 512 may beprovided with an internal flow channel to allow the cooling medium toflow efficiently behind a reflective surface. Cooling the reflectivesurfaces of the polarizers 511 and 512 efficiently and in a balancedmanner may suppress thermal deformation in the respective reflectivesurfaces. As a result, the direction and the wavefront of the pulselaser beam L1 transmitted through the optical shutter 321-4 may bestabilized. Note that a cooling device may also be provided on thePockels cell 503 to suppress overheating in the Pockels cell 503.

7. Saturable Absorber Device

The saturable absorber device of the above-described embodiments willnow be described with specific examples.

7.1 Adjusting Concentration of Saturable Absorber Gas

A saturable absorber device in which a concentration of a saturableabsorber gas can be adjusted will now be described with reference to thedrawing. FIG. 59 schematically illustrates an exemplary configuration ofsuch a saturable absorber device.

As shown in FIG. 59, a saturable absorber device 322A may include asaturable absorber gas cell 601, a heat exchanger 622, a gas temperaturecontroller 620, and a saturable absorber gas cell controller 610. Thesaturable absorber gas cell controller 610 may control each component ofthe saturable absorber device 322A in accordance with a signal from thepedestal controller 56.

The saturable absorber gas cell 601 may include windows 602 and 603,through which the pulse laser beam L1 may travel. A pressure sensor 611may be connected to the saturable absorber gas cell 601 to measure apressure inside the saturable absorber gas cell 601. The pressure sensor611 may be connected to the saturable absorber gas cell controller 610.The interior of the saturable absorber gas cell 601 may be incommunication with the heat exchanger 622 through a gas pipe 621. A gaspump 623 may be provided in the gas pipe 621 to allow the saturableabsorber gas in the gas pipe 621 to circulate between the saturableabsorber gas cell 601 and the heat exchanger 622. Further, a temperaturesensor 624 may be provided in the gas pipe 621 to detect a temperatureof the saturable absorber gas circulating in the gas pipe 621. The gastemperature controller 620 may be connected to the temperature sensor624. The gas temperature controller 620 may drive the heat exchanger 622based on a signal from the saturable absorber gas cell controller 610,to thereby control a temperature of the circulating saturable absorbergas.

The saturable absorber gas cell controller 610 may be connected to thegas pump 623. The saturable absorber gas cell controller 610 may controlthe number of revolutions of the gas pump 623, to thereby control a flowrate of the saturable absorber gas circulating in the gas pipe 621.

An SF₆ gas cylinder 631 may be connected to the gas pipe 621 throughvalves 633 and 632. Further, a buffer gas cylinder 634 may be connectedto the gas pipe 621 through valves 633 and 635. A buffer gas may be N₂,He, or the like. Furthermore, a discharge pump 637 may be connected tothe gas pipe 621 through a valve 636. The valves 632, 633, 635, and 636may be connected to the saturable absorber gas cell controller 610. Thedischarge pump 637 may also be connected to the saturable absorber gascell controller 610. The saturable absorber gas cell controller 610 mayappropriately adjust opening of the valves 632, 633, 635, and 636 andthe number of revolutions of the discharge pump 637, to thereby adjust agas pressure in the gas pipe 621 and a concentration of the saturableabsorber gas.

By adjusting a concentration of the saturable absorber gas in thesaturable absorber gas cell 601, pedestal energy of the pulse laser beamL1 passing through the saturable absorber device 322A may be adjusted.

7.2 Adjusting Optical Path Length through Saturable Absorber Gas

A saturable absorber device in which an optical path length through asaturable absorber gas can be adjusted will now be described withreference to the drawing. FIG. 60 schematically illustrates an exemplaryconfiguration of such a saturable absorber device.

As shown in FIG. 60, a saturable absorber device 322B may be similar inconfiguration to the saturable absorber device 322A shown in FIG. 59.However, the saturable absorber device 322B of FIG. 60 may include asaturable absorber gas cell 701 in place of the saturable absorber gascell 601.

As shown in FIG. 60, the saturable absorber gas cell 701 may include awindow 702, through which the pulse laser beam L1 may travel.High-reflection mirrors 711 and 712 may be provided inside the saturableabsorber gas cell 701 to bend a beam path of the pulse laser beam L1.The high-reflection mirrors 711 and 712 may be positioned so that thepulse laser beam L1 entering the saturable absorber gas cell 701 throughthe window 702 is reflected sequentially by the high-reflection mirrors711 and 712 to be outputted through the window 702.

The high-reflection mirrors 711 and 712 may be fixed to a moving stage713. The moving stage 713 may be movable along rails 714 provided in thesaturable absorber gas cell 701. The rails 714 may extend in a directionparallel to the travel direction of the pulse laser beam L1.

Further, a moving device 715 may be provided in the saturable absorbergas cell 701 to move the moving stage 714 along the rails 714. Themoving device 715 may be connected to a driver 720. The pedestalcontroller 56 may control the moving device 715 through the driver 720to move the moving stage 713, to thereby adjust an optical path lengthinside the saturable absorber gas cell 701.

By adjusting an optical path length inside the saturable absorber gascell 701, pedestal energy of the pulse laser beam L1 passing through thesaturable absorber device 322B may be adjusted.

8. Supplementary Descriptions

8.1 Diffused Target

In the preceding description, a diffused target may be a target in astate where particles containing at least one of atoms, molecules,clusters, fine droplets of a target material are diffused in a mist orgas form. A diffused target may contain a target material that in partis turned into plasma.

FIG. 61 shows a target irradiated with a pre-pulse laser beam. As shownin FIG. 61, a diffused target 271 may be generated when a target 27 isirradiated with a pre-pulse laser beam 41. When the spherical ordroplet-shaped target 27 is irradiated with the pre-pulse laser beam 41,a torus-shaped or disc-shaped diffused target 271 may be generated.

8.1.1 Generation of Diffused Target

Generation process of a diffused target will be described in detail withreference to FIGS. 62 through 64. FIG. 62 shows a target irradiated witha pre-pulse laser beam, as viewed in a direction perpendicular to thetravel direction of the pre-pulse laser beam. FIG. 63 shows a diffusedtarget generated when a target is irradiated with a pre-pulse laser beambeing irradiated with a main pulse laser beam, as viewed in a directionperpendicular to the travel direction of the main pulse laser beam. FIG.64 shows a diffused target generated when a target is irradiated with apre-pulse laser beam being irradiated with a main pulse laser beam, asviewed in the travel direction of the main pulse laser beam.

As shown in FIG. 62, when the target 27 is irradiated with the pre-pulselaser beam 41, plasma may be generated by laser ablation to a side ofthe target 27 irradiated with the pre-pulse laser beam 41. When theplasma diffuses, a shock wave S generated as the reaction to the laserablation may propagate into the target 27. As a result, the target 27may be broken up, and the diffused target 271 may be generated.

As shown in FIG. 63, the diffused target 271 may typically move with acomponent in a direction D1 the same as the travel direction of thepre-pulse laser beam 41. The main pulse laser beam 31 may be focused topass through a space including a range within which the diffused target271 moves and/or diffuses. For example, as shown in FIG. 64, a diameterDm of the main pulse laser beam 31 in the plasma generation region 25may be larger than a diffusion range Dd of the diffused target 271diffused in a torus-shape or in a disc-shape.

8.2 Relationship Between Delay Time for Main Pulse Laser Beam andConversion Efficiency

FIG. 65 shows an example of a relationship between conversion efficiencyand a delay time from the irradiation of a target with a pre-pulse laserbeam until the irradiation of the target with a main pulse laser beam.FIG. 65 shows a case where the wavelength of the pre-pulse laser beam 41is 1.06 μm, the pulse duration is 5 ns, and the fluence is 490 mJ/cm²,and where the main pulse laser apparatus 3A is a CO₂ laser, the pulseduration of the main pulse laser beam 31 is 20 ns, and the beamintensity is 6.0×10⁹ W/cm².

In FIG. 65, a line D12 shows a case where the diameter of the target 27is 12 μm, a line D20 shows a case where the diameter of the target 27 is20 μm, a line D30 shows a case where the diameter of the target 27 is 30μm, and a line D40 shows a case where the diameter of the target 27 is40 μm.

When the diameter of the target 27 is 12 μm, a delay time for the mainpulse laser beam 31 with respect to the pre-pulse laser beam 41 may bein a range of 0.5 μs to 2.0 μs. In other examples, the delay time may bein a range of 0.6 μs to 1.5 μs. In yet other examples, the delay timemay be in a range of 0.7 μs to 1.0 μs.

When the diameter of the target 27 is 20 μm, a delay time for the mainpulse laser beam 31 with respect to the pre-pulse laser beam 41 may bein a range of 0.5 μs to 2.5 μs. In other examples, the delay time may bein a range of 1.0 μs to 2.0 μs. In yet other examples, the delay timemay be 1.3 μs.

When the diameter of the target 27 is 30 μm, a delay time for the mainpulse laser beam 31 with respect to the pre-pulse laser beam 41 may bein a range of 0.5 μs to 4.0 μs. In other examples, the delay time may bein a range of 1.5 μs to 3.5 μs. In yet other examples, the delay timemay be in a range of 2.0 μs to 3.0 μs.

When the diameter of the target 27 is 40 μm, a delay time for the mainpulse laser beam 31 with respect to the pre-pulse laser beam 41 may bein a range of 0.5 μs to 6.0 μs. In other examples, the delay time may bein a range of 1.0 μs to 5.0 μs. In yet other examples, the delay timemay be in a range of 2.0 μs to 4.0 μs.

8.3 Relationship Between Fluence of Pre-Pulse Laser Beam and Shape ofDiffused Target

A relationship between a fluence of a pre-pulse laser beam and a shapeof a diffused target will now be discussed in detail with reference tothe drawings.

FIGS. 66 through 69 show a shape of a diffused target and plasmaobserved in a case where a fluence of a pre-pulse laser beam is 480mJ/cm². FIG. 66 shows a case where an elapsed time from irradiation withthe pre-pulse laser beam is 0 μs. FIG. 67 shows a case where an elapsedtime from irradiation with the pre-pulse laser beam is 0.5 μs. FIG. 68shows a case where an elapsed time from irradiation with the pre-pulselaser beam is 1.0 μs. FIG. 69 shows a case where an elapsed time fromirradiation with the pre-pulse laser beam is 1.5 μs.

FIGS. 70 through 73 show a shape of a diffused target and plasmaobserved in a case where a fluence of a pre-pulse laser beam is 96mJ/cm². FIG. 70 shows a case where an elapsed time from irradiation withthe pre-pulse laser beam is 0 μs. FIG. 71 shows a case where an elapsedtime from irradiation with the pre-pulse laser beam is 0.5 μs. FIG. 72shows a case where an elapsed time from irradiation with the pre-pulselaser beam is 1.0 μs. FIG. 73 shows a case where an elapsed time fromirradiation with the pre-pulse laser beam is 1.5 μs.

FIGS. 74 through 77 show a shape of a diffused target and plasmaobserved in a case where a fluence of a pre-pulse laser beam is 19.5mJ/cm². FIG. 74 shows a case where an elapsed time from irradiation withthe pre-pulse laser beam is 0 μs. FIG. 75 shows a case where an elapsedtime from irradiation with the pre-pulse laser beam is 0.5 μs. FIG. 76shows a case where an elapsed time from irradiation with the pre-pulselaser beam is 1.0 μs. FIG. 77 shows a case where an elapsed time fromirradiation with the pre-pulse laser beam is 1.5 μs.

Further, FIGS. 66 through 77 show a case where Sn is used as the targetmaterial, the diameter of the target 27 is 20 μm, a YAG laser is used asthe pre-pulse laser apparatus 40, and the pulse duration of thepre-pulse laser beam 41 is 5 ns. In each of FIGS. 66 through 77, thepre-pulse laser beam 41 strikes the target 27 from the right side of thepaper plane.

As shown in FIGS. 66, 70, and 74, at a point where an elapsed time fromirradiation with the pre-pulse laser beam 41 is 0 μs, that is, at thetime when the target 27 is irradiated with the pre-pulse laser beam 41,the diffused target 271 was not observed and only plasma 272 of thetarget material was observed.

As shown in FIGS. 67 through 69, when a fluence of the pre-pulse laserbeam 41 is 480 mJ/cm², the torus-shaped diffused target 271 was observedwhere an elapsed time from the irradiation with the pre-pulse laser beam41 is in a range of 0.5 μs to 1.5 μs. When an elapsed time is equal toor greater than 1.5 μs, if the spot size Dm of the main pulse laser beam31 is about 300 μm, a large portion of the diffused target 271 may beirradiated with the main pulse laser beam 31.

As shown in FIGS. 71 through 73 and 75 through 77, when a fluence of thepre-pulse laser beam 41 is 96 mJ/cm² or 19.5 mJ/cm², the disc-shapeddiffused target 271 was observed where an elapsed time from theirradiation with the pre-pulse laser beam 41 is in a range of 0.5 μs to1.5 μs. Further, as the elapsed time increases, the diffusion range ofthe diffused target 271 increased. When a fluence of the pre-pulse laserbeam 41 is 96 mJ/cm², the diffusion range of the diffused target 271 wasgreater, compared to the case when a fluence of the pre-pulse laser beam41 is 19.5 mJ/cm².

In any of the cases shown in FIGS. 66 through 77, the EUV light 252 wasgenerated when the diffused target 271 and/or the plasma 272 were/wasirradiated with the main pulse laser beam 31.

8.4 Regenerative Amplifier

FIG. 78 illustrates an exemplary configuration of a regenerativeamplifier. The regenerative amplifier 430 may include a polarizationbeam splitter 431, a CO₂ gas amplification part 432, Pockels cells 433and 436, a quarter-wave plate 434, and resonator mirrors 435 and 437.

The polarization beam splitter 431 may, for example, be formed of athin-film polarizer. The polarization beam splitter 431 may, forexample, reflect an S-polarization component and transmit aP-polarization component of a laser beam incident thereon. When thepulse laser beam L1 that largely contains an S-polarization componentwith respect to the polarization beam splitter 431 enters theregenerative amplifier 430, the pulse laser beam L1 may first bereflected by the polarization beam splitter 431. Thus, the pulse laserbeam L1 may be taken into the resonator formed by the resonator mirrors435 and 437. The pulse laser beam L1 taken into in the resonator may beamplified as it passes through the CO₂ gas amplification part 432. Thepulse laser beam L1 may pass through the Pockels cell 433, to which avoltage is not applied, be transmitted through the quarter-wave plate434, be reflected by the resonator mirror 435, and again be transmittedthrough the quarter-wave plate 434. With this configuration, thepolarization direction of the pulse laser beam L1 may be rotated by 90degrees.

Thereafter, the pulse laser beam L1 may pass through the Pockels cell433 again, to which a voltage is not applied. At this point, apredetermined voltage may be applied to the Pockels cell 433 after thepulse laser beam L1 passes therethrough. The Pockels cell 433, to whicha voltage is applied, may give a quarter-wave phase shift to the pulselaser beam L1 passing therethrough. Accordingly, while the predeterminedvoltage is applied to the Pockels cell 433, the polarization directionof the pulse laser beam L1 incident on the polarization beam splitter431 may largely include a P-polarization component with respect thereto.Thus, the pulse laser beam L1 may be trapped in the resonator.

Then, at a timing at which the pulse laser beam L1 is to be outputted, apredetermined voltage may be applied to the Pockels cell 436. The pulselaser beam L1 traveling back and forth in the resonator may betransmitted through the polarization beam splitter 431 and then besubjected to a quarter-wave phase shift when passing through the Pockelscell 436. Then, the linearly polarized pulse laser beam L1 may betransformed into the circularly polarized pulse laser beam L1.Thereafter, the pulse laser beam L1 may be reflected by the resonatormirror 437 and again pass through the Pockels cell 436. Thus, thecircularly polarized pulse laser beam L1 may be transformed into thelinearly polarized pulse laser beam L1. The pulse laser beam L1 at thispoint may largely include an S-polarization component with respect tothe polarization beam splitter 431 and be reflected thereby.Accordingly, the pulse laser beam L1 may be outputted from theregenerative amplifier 430.

The above-described embodiments and the modifications thereof are merelyexamples for implementing this disclosure, and this disclosure is notlimited thereto. Making various modifications according to thespecifications or the like is within the scope of this disclosure, andother various embodiments are possible within the scope of thisdisclosure. For example, the modifications illustrated for particularones of the embodiments can be applied to other embodiments as well(including the other embodiments described herein).

The terms used in this specification and the appended claims should beinterpreted as “non-limiting.” For example, the terms “include” and “beincluded” should be interpreted as “including the stated elements butnot limited to the stated elements.” The term “have” should beinterpreted as “having the stated elements but not limited to the statedelements.” Further, the modifier “one (a/an)” should be interpreted as“at least one” or “one or more.”

What is claimed is:
 1. A system for an extreme ultraviolet (EUV) lightsource, the system comprising a pre-pulse laser apparatus and a mainpulse laser apparatus, the pre-pulse laser apparatus configured tooutput a pre-pulse laser beam so as to turn a target into a diffusedtarget, and the main pulse laser apparatus including: a light generatorconfigured to output an optical pulse; an optical element configured totransform a waveform of the optical pulse; a controller configured todetermine a characteristic of the transformed waveform, the transformedwaveform including a first portion and a second portion, the secondportion having a temporal energy profile based on a temporal profile ofthe optical pulse and the first portion having a temporal energy profileincluding a pedestal that is different from the temporal energy profileof the optical pulse, the temporal energy profile of the first portionbeing continuously adjustable to increase and being continuouslyadjustable to decrease an energy of the pedestal independent of the restof the pulse; and an amplifier including a gain medium, the amplifierconfigured to amplify the first portion and the second portion to form amain pulse laser beam including an amplified first portion and anamplified second portion, the amplified first portion and the amplifiedsecond portion containing sufficient energy to turn the diffused targetinto a plasma that emits EUV light.
 2. The system according to claim 1,wherein the diffused target has one of a disc-shape and a torus-shape.3. The system according to claim 1, wherein the target has a dropletshape.
 4. The system according to claim 3, wherein a diameter of thetarget is equal to or greater than 12 μm and equal to or smaller than 40μm.
 5. The system according to claim 1, wherein a delay time for themain pulse laser beam with respect to the pre-pulse laser beam is in arange of 0.5 μs to 2.5 μs.
 6. The system according to claim 1, whereinthe amplifier contains CO₂ gas as the gain medium.
 7. The systemaccording to claim 1, wherein the amplified first portion has a maximumenergy that is less than a maximum energy of the amplified secondportion.
 8. The system according to claim 1, wherein a ratio of energyof the amplified first portion to total energy of the amplified firstportion and the amplified second portion is in a range of 1% to 10%. 9.The system according to claim 1, wherein energy of the amplified firstportion is in a range of 1 mJ to 10 mJ.
 10. The system according toclaim 1, wherein the optical element includes a Pockels cell.
 11. Asystem for an extreme ultraviolet (EUV) light source, the systemcomprising a pre-pulse laser apparatus and a main pulse laser apparatus,the pre-pulse laser apparatus configured to output a pre-pulse laserbeam so as to turn a target into a diffused target, and the main pulselaser apparatus including: a light generator configured to output anoptical pulse including a portion to be transformed and a portion to betransmitted without being transformed, the portion to be transformedcontinuously gradually rising on average to the portion to betransmitted; an optical element configured to transform a waveform ofthe optical pulse; a controller configured to determine a characteristicof the transformed waveform, the transformed waveform including a firstportion and a second portion, the second portion having a temporalenergy profile based on a temporal profile of the optical pulse and thefirst portion having a temporal energy profile including a pedestal thatis different from the temporal energy profile of the optical pulse thetemporal profile of the first portion being continuously adjustable toincrease and being continuously adjustable to decrease an energy of thepedestal independent of the rest of the pulse; and an amplifierincluding a gain medium, the amplifier configured to amplify the firstportion and the second portion to form a main pulse laser beam includingan amplified first portion and an amplified second portion, theamplified first portion and the amplified second portion containingsufficient energy to turn the diffused target into a plasma that emitsEUV light.
 12. The system according to claim 11, wherein the diffusedtarget has one of a disc-shape and a torus-shape.
 13. The systemaccording to claim 11, wherein the target has a droplet shape.
 14. Thesystem according to claim 13, wherein a diameter of the target is equalto or greater than 12 μm and equal to or smaller than 40 μm.
 15. Thesystem according to claim 11, wherein a delay time for the main pulselaser beam with respect to the pre-pulse laser beam is in a range of 0.5μs to 2.5 μs.
 16. The system according to claim 11, wherein theamplifier contains CO₂ gas as the gain medium.
 17. The system accordingto claim 11, wherein the amplified first portion has a maximum energythat is less than a maximum energy of the amplified second portion. 18.The system according to claim 11, wherein a ratio of energy of theamplified first portion to total energy of the amplified first portionand the amplified second portion is in a range of 1% to 10%.
 19. Thesystem according to claim 11, wherein energy of the amplified firstportion is in a range of 1 mJ to 10 mJ.
 20. The system according toclaim 11, wherein the optical element includes a Pockels cell.